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Daylight and health: A review of theevidence and consequences for thebuilt environmentMBC Aries PhD, MSca,c, MPJ Aarts MSca,c and J van Hoof PhD, MSc, Eur Ingb

aDepartment of the Built Environment, Eindhoven University of Technology, Eindhoven,the NetherlandsbCentre for Healthcare and Technology, Fontys University of Applied Sciences,Eindhoven, the NetherlandscIntelligent Lighting Institute, Eindhoven University of Technology, Eindhoven, theNetherlands

Received 2 July 2013; Revised 20 September 2013; Accepted 24 September 2013

Daylight has been associated with multiple health advantages. Some of these claimsare associations, hypotheses or beliefs. This review presents an overview of ascientific literature search on the proven effects of daylight exposure on humanhealth. Studies were identified with a search strategy across two main databases.Additionally, a search was performed based on specific health effects. The results arediverse and either physiological or psychological. A rather limited statisticallysignificant and well-documented scientific proof for the association betweendaylight and its potential health consequences was found. However, the searchbased on specific health terms made it possible to create a first subdivision ofassociations with daylight, leading to the first practical implementations for buildingdesign.

1. Introduction

Humans have evolved under the influence ofdaylight and the light–dark cycle. On the onehand, the human skin provides a layer ofpigmentation to protect us from the highestradiation intensities when exposed to daylightalmost every day. On the other hand, humanshave developed a variety of physiologicalresponses to the varied characteristics ofdaylight. Daylight was the main light sourceuntil electric lighting became reliable andaffordable. Since the introduction of electriclighting, a large part of the population startedspending most of its time inside buildings. It

sometimes even appears as if daylight hasonly an architectural value, and all otherdaylight functions have been replaced byelectrical lighting solutions.

Solar radiation is filtered through theatmosphere and radiation reaching theEarth’s surface is mainly in the wavelengthrange 200–4000 nm; some visible, some invis-ible to the human eye. The portion of thespectrum to which the eye is sensitive –commonly referred to as light – is electro-magnetic radiation with a wavelength in therange from about 380 nm to about 780 nm.Radiation with wavelength between 100 nmand 400 nm is called ultraviolet (UV) radi-ation and is usually divided into UV-C (200–280 nm), UV-B (280–315 nm) and UV-A(315–400 nm). Radiation with wavelengthbetween 780 nm and 1mm is called infrared(IR). UV and IR are invisible to the human

Address for correspondence: MBC Aries, Department of theBuilt Environment, Eindhoven University of Technology, DenDolech 2, 5612 AZ Eindhoven, the Netherlands.E-mail: [email protected]

Lighting Res. Technol. 2015; Vol. 47: 6–27

� The Chartered Institution of Building Services Engineers 2013 10.1177/1477153513509258

at Charles Darwin University on March 20, 2015lrt.sagepub.comDownloaded from

http://lrt.sagepub.com/

eye. Daylight is the solar radiation, visible tothe human eye, emitted by the sun andperceived during daytime. The duration ofdaytime depends on our location on Earthand the time of year. Since daylight cannot beartificially replicated, it is often referred to asnatural light.

Humans overwhelmingly prefer workingand sitting near windows.1 However, nobodycan fully explain why. Potential reasons arethe link with the view outside with its inex-haustible supply of information, the quantityof daylight (both high and low), the presenceof the full continuous spectrum, the (changein) directionality and/or the dynamics frommilliseconds to months. Daylight providesvariety and stimulation during the day and itis widely believed that access to daylightreduces stress and increases productivity.2,3

Weather in general is found to influencepeople’s health and mood.4–7 In the multi-variate study of Denissen et al.,4 the effects ofsix weather parameters (temperature, wind,sunlight, precipitation, air pressure andphotoperiod) on mood (positive affect, nega-tive affect and tiredness) were examined. Theresults revealed important effects of tempera-ture, wind and sunlight, with sunlight alsoshowing a mediating role.

Daylight, however, because of its variabil-ity, intensity and thermal component, canalso lead to serious problems. It can cause anuncomfortable level of glare,8,9 or it makesthe building demand excessive amounts ofcooling/heating energy if too much/little radi-ation enters the building. When daylight is thecause of thermal or visual discomfort, theoccupants’ wish for daylight is diminished.Additionally, people do not switch electriclighting off when there is enough daylight.This suggests that daylight is not superior, butelectric lighting is limited in creating neces-sary variation, the provision of a view andspace illumination.10 Besides, people’s prefer-ence for daylight may be partly due to theirnegative view of electric lighting.

Radiation is increasingly administered andstudied as a non-pharmacologic treatment fora variety of health-related problems, includ-ing skin problems (UV-radiation treatment),seasonal affective disorder (SAD), depression,jetlag, as well as circadian rhythm sleepdisturbances and behavioural problems.11

Light therapy consists of exposure to daylightor to specific types of electric lighting.Exposure is prescribed for a specific durationand time of day. A little more than 50 yearsago, it was quite common for sunlight to beprescribed as part of the treatment of tuber-culosis in sanatoria.12

The World Health Organization defineshealth as ‘a state of complete physical, mentaland social well-being and not merely theabsence of disease or infirmity’.13 Daylight iswidely believed to influence human health.Daylight and daylighting have been asso-ciated with lower absenteeism, reducedfatigue, relief of SAD, decreased depressivesymptoms, improved skin conditions, bettervision, positive impact on the behaviouraldisturbances seen in Alzheimer’s disease andmultiple other health advantages. Some, buthopefully not all of these claims are associ-ations, hypotheses or beliefs. Therefore, therationale of this paper is to present anoverview of studies on the proven effects ofdaylight exposure on human health, since‘light is the most important environmentalinput, after food, in controlling bodily func-tions’.14 Moreover, we discuss the conse-quences and applicability of the results ofthe literature review for the construction andthe renovation of buildings from a practicaland architectural point of view.

2. Methodology

2.1. Search process

Proven health effects of daylight wereexamined on the basis of existing literatureand the search followed a two-step process.First, studies were identified with a search

Daylight and health: A review 7

Lighting Res. Technol. 2015; 47: 6–27



strategy across two (English language) litera-ture databases: PubMed and Scopus. InScopus, the initial search terms in the articletitle, abstract or keywords were: ‘daylight’,‘sunlight’ and ‘natural light’, in combinationwith ‘health’. In PubMed, ‘daylight’ wasincluded in all fields and ‘health’ was enteredas MeSH Term. Species was set to ‘human’(PubMed) or the word ‘human’ was includedin the search term (Scopus). In order toeliminate most results related to daylightsaving time, results with ‘saving’ and ‘acci-dent’ (all fields) were excluded, and to elim-inate most dental results, terms such as ‘oral’(all fields) were excluded, and to eliminateresults related to fasting during Ramadan,‘Ramadan’ (all fields) was excluded. Table 1shows the exact search terms used.Bibliographies of selected articles werescreened for other relevant articles. Second,searches were performed based on ‘daylight’and a specific health effect (for instance,headache), since it could be that only thisspecific term was used in the article instead of‘health’.

2.2. Inclusion and exclusion criteria

Included were published studies of daylighteffects on human health. Actual eligibility wasassessed by reading abstracts and, if neces-sary, whole articles. Due to the large amountof hits in Scopus, a pre-selection based on the

journal title (e.g. publications in the Journalof Accident Analysis and Prevention or theJournal of Public Health Dentistry wereexcluded) or topic (e.g. ‘fasting duringRamadan’) was made prior to the eligibilityprocess of studies.

2.3. Data extraction

The following data were extracted from thestudies if available: (1) studied health effect(s);(2) light source (daylight only or a combin-ation of daylight and electric lighting); (3)illuminance (including direction if possible);(4) time of exposure (either time at which theexposure occurred or the duration of theexposure); (5) number of subjects; (6) type ofstudy; (7) statistical evidence (including testdetails and significance level) and (8) conclu-sions related to daylight and health. Studyquality other than completeness of requesteddata was not further assessed.

2.4. Limitations

The study was limited to daylight only(visible radiation 380–780 nm). Known intra-and interpersonal differences (i.e. gender,photoperiod sensitivity, and daily andmonthly rhythms) were not specificallyincluded in this literature search. This alsoapplies for potential health interaction effects,and the results or interactions due to electriclighting.

Table 1 Search terms within the databases ‘PubMed’ and ‘Scopus’ (Date of last search: 20 September 2013)

Search terms

PubMed (daylight[Title/Abstract] AND health[Title/Abstract]) NOT saving[All Fields] NOT (‘oral’[All Fields]) NOT(‘Ramadan’[All Fields]) NOT (‘accident’[All Fields]) AND (hasabstract[text] AND ‘humans’[MeSHTerms] AND English[lang])

PubMed (natural light[Title/Abstract] AND health[Title/Abstract]) NOT saving[All Fields] NOT (‘oral’[All Fields])NOT (‘Ramadan’[All Fields]) NOT (‘accident’[All Fields]) AND (hasabstract[text] AND ‘humans’[MeSHTerms] AND English[lang])

Scopus TITLE-ABS-KEY(daylight AND health AND NOT saving AND NOT oral AND NOT Ramadan AND NOTaccident) LANGUAGE(English) AND (LIMIT-TO(DOCTYPE, ‘ar’) OR LIMIT-TO(DOCTYPE, ‘ip’)) AND(LIMIT-TO(EXACTKEYWORD, ‘Humans’))

Scopus TITLE-ABS-KEY(‘natural light’ AND health AND NOT saving AND NOT oral AND NOT Ramadan ANDNOT accident) LANGUAGE(English) AND (LIMIT-TO(DOCTYPE, ‘ar’) OR LIMIT-TO(DOCTYPE, ‘ip’))AND (LIMIT-TO(EXACTKEYWORD, ‘Humans’))

8 MBC Aries et al.




3. Results

In this section, an overview is given of healtheffects related to daylight exposure. Aftercomparing and pre-selecting the literaturesearch results (Table 2), an in depthabstract-based selection showed that 18unique studies from both literature databasesseem to be eligible and were further analysed.Subsequently results of additional studies(and their limitations) are reported.

3.1. Step 1: Databases ‘PubMed’ and ‘Scopus’

Table 3 shows the results from the searchwithin the PubMed and Scopus databases.Mottram et al.15 reported the best sleeptiming, duration, efficiency and qualityunder natural light conditions. The studyincluded questions and measurements (bothactigraphy and lighting measurements) andused daylight as a control condition (3-weekperiod at the beginning and end of theAntarctic winter).

Four studies focused on the relationshipbetween daylight hours and physical activity.Three studies16–18 found no significant effects,while the fourth study19 found a significantassociation. Next to the difference in meas-urement devices between those four studies(pedometers vs. accelerometers), there wasalso a difference in the amount of daylighthours per day (from 8.7 hours to 15.1 hours).The studies of Feinglass et al.19 and Klenket al.18 both compared long (15–16 hours) toshort (9 hours) photoperiods using acceler-ometers and found contradictory results.However, corrections for additional weather

parameters and the fact that the group ofFeinglass et al.19 suffered from arthritis canexplain the difference. Since significant (notclinically meaningful) results were foundbetween days with less sunlight and arthritispain severity, this could also be an explan-ation for the difference in activity level. It isgenerally not clear from the existing studies ifthe mixed results are due to limited statisticalpower (such as small sample sizes and vari-ability in weather indices).

Bodis et al.20 used also daylight hours tostudy the effect on heart attack and infarc-tion. They found a (weak) negative correl-ation: the more daylight hours, the lessinfarctions. They also found a positive cor-relation between timing and infarctions. Theinfluence of daylight hours was investigatedby Hansen et al.21 and Murray and Hay22, aswell in relation to SAD and mental distress.Both concluded that the (self-reported)depression was most likely not photoperiodspecific, since ‘human seasonality may have abroader psychological component’.22 Thispreliminary conclusion seems consistent withBjorksten et al.23 who tried to relate thedaylight photoperiod to suicide levels.Surprisingly the suicide rate in Greenlandpeaked in midsummer and was lowest in theperiod with the least daylight hours (winter).

Since month of birth can influence people’slife after, Jewell et al.24 researched the seasonof birth with length of day as a representativevariable and postpartum depression. Theyfound no significant relationship.

Electric lighting at night and daylightphotoperiod were linked to breast cancer by

Table 2 Hits per search term for the databases ‘PubMed’ and ‘Scopus’ (Date of last search: 20 September 2013)

Source Search term ‘daylight’or ‘natural light’

Hits Eligible after pre-selection

PubMed Daylight 56 16PubMed Natural light 12 4Scopus Daylight 42 20Scopus Natural light 23 7





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Davis et al.25 Exposure levels for either lightsource were not mentioned. More hours ofdaylight and subsequently the less hours ofdarkness were associated with lower noctur-nal urinary 6-sulphatoxymelatonin (aMT6s)levels. 6-sulphatoxymelatonin is the metabolicend product of the hormone melatonin.

Three other studies26–28 did not mentionthe light levels in their methodology sectioneither. They investigated health-related qual-ity of life, self-reported sleep latency and (riskfor) depression, respectively. Grimaldi et al.27

found positive results for poor indoor illu-mination and an increased mental ill-being intheir regression analysis. However, it is notclear what the exact contribution of daylightwas to this indoor illumination.

Alimoglu and Donmez29 based their day-light exposure on questionnaire results (cate-gories 51 hour, 1–3 hours and 43 hours).They investigated the link between burn-out,a psychological term for the experience oflong-term exhaustion and diminished interest.Since daylight has an impact on humanalertness and cognitive responses, Alimogluand Donmez29 investigated if daylight expos-ure in a work setting could be placed amongthe predictors of job burn-out, but found nodirect effect. They did find an indirect effectvia work-related stress and job satisfaction.More daylight exposure leads to less stress

and higher satisfaction. If this effect is exclu-sively related to daylight is not proven, sincefor example Newsham et al.30 found alsopositive correlations between the (satisfactionwith) lighting (daylight and electric lighting)and job satisfaction.

Vreeburg et al.31 researched a combinationof factors (both sampling factors and healthindicators) and found that, amongst all,sampling month (daylight hours) influencedsalivary cortisol levels. Cortisol is importantfor the hypothalamic–pituitary–adrenal(HPA) axis regulation: ‘The HPA-axis ishypothesized to be one of the key biologicalmechanisms underlying several stress-relateddisorders, including somatic and psychiatricdisorders’.31

3.2. Step 2: Specific health keywords

Table 4 shows the results from the searchon specific health keywords. The specifichealth issues with an association with daylightare divided into three categories: ‘positive’,‘negative’ and ‘both positive and negative’.Scientific literature sources were obtained viaPubMed, Scopus/ScienceDirect, GoogleScholar or at an author’s personal website.

The most well-known effect of light is onvision. Human day vision (photopic) isregulated by three cone photoreceptors,while vision in dim light (mesopic) is

Table 4 Specific health associations linked to (interaction with) daylight

Positive association Negative association Positive/negative association

Improvement of vision (and reduction ofdepression)

Triggering of migrainesTriggering of epilepsy

Influence body height and birthweight

Reduction of myopiaReduction of eyestrain (and improvement of

Increase chance for autism Influence bilirubin levels andhaem catabolism

relaxation)Reduction of headaches

Influence sleep problems forpeople with autism

Stimulation of circadian physiology andcognitive performance

Induce/modify changes inhuman gonadal function

Improving sleep qualityReduction of ADHD prevalence

Influence breast cancer tumours

Reduction of SAD depressionsPrevention of obesity

ADHD: attention-deficit/hyperactivity disorder; SAD: seasonal affective disorder.





controlled by cone and rod photoreceptors,and (almost complete) darkness (scotopic) isregulated by rod photoreceptors. This lightinput then triggers a response through theoptic nerve to the visual cortex in the brain.The primary transition between scotopic,mesopic and photopic vision – the switchfrom employing solely rod to solely conephotoreceptors – is a direct response toenvironmental irradiance.32 Human visionunder daylight conditions is normally betterthan under electric lighting due to the higherquantity (and often a better colour renderingindex), enabling better visual performance.Recently, Zhang33 concluded that self-reported visual function loss, rather thanloss of visual acuity, is significantly associatedwith depression. This study was based on across-sectional, nationally representativesample of adults 20 years of age or older(N¼ 10 480). It was not possible from thisanalysis to determine whether depression is acause or an effect of visual function loss.

Even though myopia (short-sightedness ornear-sightedness) can be corrected withglasses, contact lenses and refractive surgery,according to Morgan et al.34 it has emerged asa public health concern since its prevalence isincreasing in Asia, North America andEurope. Surveys have shown that increasedamounts of time outdoors protect against thedevelopment of myopia35,36. In a cross-sec-tional study of two age samples from 51Sydney schools, children and their parentscompleted detailed questionnaires on activityand the children had a comprehensive eyeexamination. The researchers concluded thathigher levels of total time spent outdoors wereassociated with less myopia (p¼ 0.04) andsuggested that light intensity may be animportant factor. Due to the higher lightlevels pupils will be more constricted out-doors, resulting in a greater depth of field andless image blur.35 Periods of 5–7.5 hours ofelevated light levels (15 000–28 000 lx) havebeen found to reduce the amount of myopia

in different animal species36. Norton andSiegwart36 concluded in their review that‘retinal dopaminergic activation seems verylikely to play a role in the protective effects ofoutdoor activities in children and the effectsof elevated light levels in the animal studies’.

The most common health effect operatingthrough the visual system is eyestrain.Eyestrain is pain and fatigue of the eyes,due to tightening of the ciliary muscle.37

Cowling et al.38 found that there were signifi-cantly less incidents of eyestrain reported bypeople whose workstations received largeproportions of natural light. A total of 310questionnaires were distributed in nine differ-ent buildings and 254 were returned (responserate 82%). Both chi-squared tests and mul-tiple regression analysis were employed.Headaches, severe fatigue and eyestrain werethe three conditions canvassed as havingsome work environment precursor. Themajority of respondents reported sufferingfrom all three symptoms, at least occasion-ally. The triggering source for eyestrain canbe electric equipment, lighting or daylight,although the view that comes with a daylightopening can provide a point of relaxation forthe eyes (focus in the distance) and higherincident light can reduce the pain.37 Eyestrainis often accompanied by headache, resultingfrom prolonged use of the eyes, uncorrecteddefects of vision or an imbalance of the eyemuscles.2,37,38 The decrease in headache inci-dence with daylight illuminance increase wasassessed in the study of Wilkins et al.37 usinga Jonckheere non-parametric trend test basedon the data of N¼ 20 people and correctedfor age and seniority (superiority). The illu-mination from daylight increased with theheight of the office above the ground by anaverage of 80 lx per storey (measured at thework surface on a sunny day). Headachestended to decrease with increasing storey level(z¼ 2.13, p50.02, one-tailed, before lightingchange). Robertson et al.39 compared twobuildings and 106 out of 109 (97%) workers

16 MBC Aries et al.




completed a health questionnaire. Theresearchers found a significantly higher preva-lence of work-related headache in the buildingwith less daylight and lower mean luminancesand illuminances of the work positions (evenwith electric lighting on) compared with theother building (p50.001). The building withless daylight was air conditioned and theheadache could therefore be related to eitheran interaction between daylight and ventila-tion or related to the ventilation type only.

Migraine is a recurrent moderate to severeheadache. The triggering of migraine ishypothesized to start in the visual cortex.Amongst other things, people who sufferfrom migraine are more sensitive to lightthan other people (which is also described asphotophobia).37,40 The high level of daylightand often occurring large contrast causingglare makes this light source a potentialtrigger for migraine.2 Results of a reviewedstudy in Mulleners et al.41 indicated thatpatients with migraine, both with and withoutan aura, have lower thresholds for visualstress than control subjects. Daylight, espe-cially in the window zone, usually providesmuch higher light levels than electric lighting.

Photosensitive epilepsy (PSE) is a form ofepilepsy in which seizures are triggered byvisual stimuli that form patterns in time orspace. PSE can start due to lamp flicker (witha frequency of 15 Hz), but this frequency isnot dominantly present in daylight. If day-light enters a space through a moving filter,PSE can occur.2 For example, daylight shin-ing off water or through the leaves of treescan trigger seizures. In their book, Hardingand Jeavons42 show multiple cases and studieswhere seizures had been precipitated byflickering sunlight.

People with autism have a chronically highlevel of arousal and high levels of daylight arearousing.2,43 The variability of daylight cancreate a stimulating environment, which formost people would be preferable, but not forpeople with autism. However, exposure to

daylight’s seasonal variation has positiveinfluences on people with autism. Hayashi44

reported in a case report on seasonal changesin sleep problems and behavioural problemsin an adolescent with autism over the year.Sleep problems decreased from January toJune, and disappeared in July and August.Most of the behavioural problems (i.e. crying)decreased gradually from January to June.The subject was one 15-year-old autistic male.Recently, Mazumdar et al.45 showed evidenceof seasonality in the risk of conceiving a childlater diagnosed with autism. They authorsapplied a one-dimensional scan statistic (withadaptive temporal windows) on case andcontrol population data from California,USA for the years 1992 through 2000(with4400 000 births per year).

Foster and Roenneberg46 state that ‘despitehuman isolation from seasonal changes intemperature, food and photoperiod in theindustrialized nations, the seasons still appearto have a small, but significant, impact uponwhen individuals are born and many aspectsof health’. Using a large US human malepopulation of 507 125 people, Weber et al.47

found clear evidence for a dependence ofbody height at age 18 on birth month. Over aperiod of 10 years there is a sinusoidalvariation with a period of 1.0 year withmaxima in spring and minima in autumndiffering by 0.6 cm, a difference of 0.3%associated with a changing photoperiod(height M� SD¼ 177.2� 0.33 cm with0.057 cm/year secular trend; sunshine dur-ation M� SD¼ 144� 65 hours with a trendof 10.69 hours/year). They linearly interpo-lated both datasets, generated a Fourierspectrum and produced a Lomb–Scargleperiodogram. The authors cannot offerdefinitive explanations but hypothesize thatthe underlying physiological mechanismmight involve the light-dependent activity ofthe pineal gland. Also Wohlfahrt et al.48

found a circannual variation in length at birthin a population-based cohort of 1 166 206





children born in Denmark. The circannualvariation of 2.2mm in length at birth iscompatible with the 6mm variation of Weberet al.47 Additionally, they showed that dis-crepancies in measurements over the seasonsare not likely, since they also found seasonalvariations in birth weight, which is often moreaccurately measured. Details related to pro-cedures and statistical outcomes are notmentioned. Seasonal variations were alsofound related to bilirubin levels in newborns.Bilirubin is the yellow breakdown product ofnormal haem catabolism occurring naturallywhen red blood cells die. Anttolainen et al.49

found a significantly lower bilirubin valuefrom the fifth day of life onwards in a groupof Finnish infants born during the light halfof the year (maximum of 22 hours ofdaylight), compared with infants bornduring the dark half of the year (maximumof 3 hours of daylight). In total, 86 preterminfants born consecutively during one calen-dar year were studied.

Many aspects of human physiology andbehaviour are adapted to the 24-hour light/dark cycle generated by the Earth’s rotation.This 24-hour rhythm has a major impact onhuman health and well-being,50 and all per-ipheral organs have autonomous, light-responsive oscillators. The 24-hour, or circa-dian, clocks use daylight to synchronize(entrain) to the organism’s environment.51

Studies from Roenneberg et al.52 stronglysuggested that the human circadian clock ispredominantly entrained by sun time ratherthan by social time. In 2001, two researchgroups53,54 used the effect of light to suppressnocturnal human melatonin secretion as amarker of an effect on the circadian system.The observed action spectrum for melatoninsuppression showed short-wavelength sensi-tivity very different from the known spectralsensitivity of the scotopic and photopicresponse curves. The non-visual alertingeffects of light during night time appear to berelated to melatonin suppression.55 The

alerting effects during the daytime (whenmelatonin is not present) occur through dif-ferent pathways. According to Cajochen,56 it ismore likely to be the ventromedial preopticarea. Not only blue light (�max¼ 460 nm) butalso green light (�max¼ 555 nm) elicits non-visual responses to light, such as resettingcircadian rhythms, suppressingmelatonin pro-duction and alerting the brain.57 The sensitiv-ity of the human alerting and cognitiveresponse to polychromatic light at levels aslow as 40 lx is blue-shifted relative to the three-cone visual photopic system.58 Daylight inten-sity is most of the day much higher than 40 lxand will certainly have a significant impact oncircadian physiology and cognitive perform-ance (alertness). It also contains the fullspectrum, with changing composition overthe day. Daylight not only has its impactduring the day but also at night daytime lightexposure can play a role. Recently, Cheunget al.59 reported their results of workplacedaylight exposure on sleep quality (PittsburghSleep Quality Index), physical activity andquality of life. Employees (N¼ 49) with awindow in their workplace got significantlymore natural light exposure (p50.05) andtheir actiwatches registered on average 47minutes more sleep (p50.05).

Arns et al.60 studied the relationshipbetween the prevalence of attention-deficit/hyperactivity disorder (ADHD) and solarintensity (SI) on the basis a cross-state(study 1) and multinational study (study 2).In the datasets, a significant relationshipbetween SI and the prevalence of ADHDwas found (Study 1: 2003: p50.000;r2¼ 0.637, 34% variance explained; 2007:p50.000; r2¼ 0.580; 41% variance explained;Study 2: p¼ 0.018; r2¼�0.758, 57% varianceexplained). Approximately, 80% of adultADHD patients and one-third of childrenwith ADHD suffer from sleep onset insom-nia, characterized by a delayed circadianphase and delayed melatonin peak, whichcould be the result of increased use of modern

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(social) media (iPads, mobile phones), espe-cially shortly before bedtime. According tothe researchers ‘the apparent preventativeeffect of high SI [solar intensity] on ADHD[attention-deficit/hyperactivity disorder] pre-valence might thus result from the ability ofintense natural light during the morning tocounteract the phase delaying effects ofmodern media in the evening, thus preventingthe delayed sleep onset and reduced sleepduration’.60

Seasonal changes in day length (photo-period) and linked night length (scotoperiod)induce changes in the duration of melatoninsecretion at night. The duration of nocturnalmelatonin secretion is longer in winter thansummer and triggers seasonal changes inbehaviour.61–63 In general, alterations inmonoaminergic neurotransmission in thebrain are thought to underlie seasonal vari-ations in mood, behaviour and affective dis-orders.64 SAD is a syndrome characterized byrecurrent depressions that recur every autumn/winter. The lack of sufficient natural daylightin winter is often thought to be the reasonbehind SAD.63,65 The reduction of depressiondue to exposure to daylight is not fullyunderstood yet. Several researchers haveshown that the prevalence of self-reporteddepression was surprisingly low in winter(SAD-season) considering the lack of day-light.21,22,66 The study of Lambert et al.64

showed that the turnover of the monoamineneurotransmitter serotonin by the brain waslowest in the Australian winter (non-SAD-season). Serotonin has a role in the develop-ment of seasonal depression. The rate ofproduction of serotonin by the brain wasdirectly related to the prevailing duration ofbright sunlight (r¼ 0.294, p¼ 0.010), but it wasnot related to the hours of sunlight on the daybefore the study. The authors also found that,irrespective of the month of the year, turnoverof serotonin in the brain was affected by acutechanges in light intensity, with values beinghigher on bright days than on dull days.64

Season of the year is known to affect thenocturnal rise in melatonin67. Melatonin isinvolved in a variety of diseases, includingcancer, insomnia, depression, dementia,hypertension and diabetes. The daylightphotoperiod was specifically linked to breastcancer. Women with malignant tumoursappeared to have significantly lower 24-hourconcentrations of aMT6s (6-sulphatoxymela-tonin) compared with women with benigntumours. A study by Obayashi et al.68 showedthat daylight exposure (at least 1000 lxbetween 37 and 124 minutes, mean 72 min-utes) in an uncontrolled daily life setting ispositively associated with urinary 6-sulpha-toxymelatonin excretion in the elderly.

Environmental lighting can induce ormodifychanges in human gonadal function. A studywith blind versus non-blind girls showed thatpuberty developed earlier than normal in blindgirls. In a study with rats, nocturnal animals,puberty developed later than expected in blindlaboratory rats.Thedifferencewas explainedbythe fact that humans are active diurnally.69 Amore recent statistical analysis by Flynn-Evanset al.70 was conducted to determine whetherdifferences exist in reproductive measuresamong blind women (N¼ 1392) with at leastlight perception (LP) compared with womenwith no perception of light (NPL) in a cohortstudy. Student’s two-sample t-tests and multi-variate logistic or linear regression were con-ducted to get statistical results. The findingssuggested as well that lack of LP affectsreproductive development in women (odd’sratio NLP vs. LP from birth was 0.88; 95%).A parallel study based on the same group ofwomen by Flynn-Evans et al.71 used multi-variate-logistic regression models. Theseshowed that blind women with NPL appear tohave a lower risk of breast cancer, comparedwith blind women with LP (odds ratio, 0.43;95%), the indirect effect of light may go farbeyond the influence on glandular functionsonly, potentially with a role for urinary6-sulphatoxymelatonin and melatonin.





Regular physical activity is crucial forhuman health and it stimulates the level andduration of independence of older people.Weather is widely believed to influencepeople’s health, mood and their physicalactivity level. Particularly among olderpeople, physical activity levels are muchhigher in summer than in winter. Daylength, sunshine duration and maximumtemperature have a significant influence onphysical activity levels.72 Brown adiposetissue (BAT) is present in adult humans andmay be important in the prevention of obes-ity. The study of Au-Yong et al.73 demon-strated a very strong seasonal variation in thepresence of BAT relating to ambient tem-perature and photoperiod. This effect wasmore closely associated with photoperiod(r2¼ 0.876) than ambient temperature(r2¼ 0.696). The authors studied 3614 con-secutive patients and performed a �2 test.

4. Discussion

4.1. The influence of daylight on health: The

scientific evidence

Humans have evolved under the influenceof daylight and its light–dark cycle. This isprobably why people believe that daylight ispositively related to human health. Some ofthe found and investigated studies reportedresults on ‘general health’. More specifichealth issues reported are either physiological(work-related headache, activity level, heartattack/myocardial infarction, insomnia andbreast cancer) or psychological (depression,burn-out, SAD, mental distress and suicide).Objective health measurements that are usedare ‘activity’ (by means of an accelerometer,actiwatch or pedometer), ‘salivary cortisol’(samples) and database contents regarding‘heart attack’ and ‘suicide’. The results foundwhen searched for more specific relations arealso either physiological (visual acuity, eye-strain, headache/migraine, epilepsy, autism,body height, birth weight, bilirubin levels,

serotonin levels, human gonadal functioning,breast cancer and obesity) or psychological(alerting effects, burn-out and SAD). The factthat effects of daylight were more frequentlyfound when searching under specific medicalconditions suggests that much of the currentliterature is aimed at solving medical condi-tions instead of providing healthy indoorenvironments.

The found studies in the two databasessearch were rather limited. It was expected tofind more studies. Moreover, different scien-tific proof regarding daylight and healtheffects was actually found by searching oneffects directly, which shows there may be amissing link in choice of words for titles,abstracts, search options or key words.

The studies in the two databases were allchecked for several information elements,necessary to assess the initial quality of thestudy. The results show that all but one of theselected studies reported on the used methodsand statistical outcomes. It was striking thatilluminances or light exposure were only veryoccasionally documented. Only one paper, byMottram et al.15, reported actual illuminanceswith regard to daylight exposure. The lack ofdaylight levels makes it hard to find aconsistent conclusion regarding daylightinfluence, especially since the intensity andduration of daylight changes over the day andyear. Also the distinction between exclusivedaylight exposure or a combination of day-light and electric lighting is not documented,which makes a conclusion relating the effectof daylight impossible.

Multiple studies have found a significantinfluence of the difference in daylight hoursper day (photoperiod). The focus of allstudies was on daylight in general or thephotoperiod specifically. No research wasfound related to the dynamics of daylight,other than day length.

Multiple studies used techniques that focuson obtaining subjective results (self-reportedhealth effects, SAD-questionnaire answers,

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etc.). In some cases, questions were notspecifically designed for daylight and healthresearch, but were part of a more extensivequestionnaire related to general health. Thequestionnaires reported are, therefore, verydifferent, and in most cases a copy of thequestions was not available via the paper.Some studies reported results on ‘generalhealth’. No specific questionnaire focusingon daylight and health was found.

Most found studies were executed withdaylight as light source; some studies used acombination of light sources (daylight andelectric lighting). However, conclusions arenot always exclusive to daylight only. Forinstance, studies that prove that daylightmakes people more or more efficiently alertthan electric lighting or show the effect ongonadal functioning and breast cancer exclu-sively related to daylight rather than toelectric lighting do not exist (yet).

4.2. Daylight quality

In 1929, the French architect Le Corbusiersaid that ‘the history of architectural mater-ial . . .has been the endless struggle forlight . . . in other words, the history of win-dows’. Most architects are devoted to daylightsince they know that no other building com-ponent has such a significant impact on theirdesign of a building than daylight openings.

People in the Western world spendapproximately 80–90% of their time indoorsand therefore buildings play an importantrole in providing a healthy daylight environ-ment. Daylight exposure outdoors means fullexposure to solar radiation with all possiblepositive and negative health effects. Indoors,people’s exposure is basically limited to vis-ible and IR radiation, even though glassinnovations attempt to limit the IR contribu-tion significantly due to thermal discomfort.The design of the building and its floor planslargely dictates how the building can and willbe used. Humans overwhelmingly preferworking, learning and sitting near daylight

openings,1 provided thermal or visual dis-comfort are absent. The current design ofbuildings does not allow this for all users.

The reason why people prefer a window seatcannot entirely be explained. It is unknownwhether there is a connection or associationwith health or comfort effects. Potential rea-sons are the relationship with the view outsidewith its inextricable supply of information/view, the quantity of daylight (both high andlow), the presence of the full continuousspectrum, the (change in) directionality and/or the dynamics from milliseconds to months.The maximal seating distance to the windowfor a good daylight experience is not known.

The dynamics in daylight availability varyfrom months to milliseconds. Many healtheffects are stimulated via ocular light expos-ure, and the origin of the trigger (i.e. photo-period) can be far in the past (i.e. before or atbirth). The brain structures and functions tomeasure changes in day length are still presentin humans, though mostly not directly appar-ent. Much stronger is the existence of acircadian rhythm as manifested by the sleep/wake cycle. The endogenous rhythm of thehuman body clock is usually slightly longerthan 24 hours and thus needs a daily morninglight signal to reset the clock to entrain withthe Earth’s 24-hour rotation rhythm and thechanging photoperiod. Health effects as aresult of different levels of daylight variationsare largely unknown. Additionally, variationsin light dynamics are introduced by lighting incomputer screens, electric lighting or lightingand shading controls. These manmade fre-quencies can support, substitute or counteractdaylight frequencies, and therefore trigger orreduce health effects. Certain effects of day-light are related to the moment of birth andthe photoperiod. These effects have no con-sequences for the design of the built environ-ment, but demonstrate potential unknowninfluences of the changing photoperiod.

The daylight spectrum represents all wave-lengths of the solar visual spectrum. Studies





related to isolated parts of the sunlightspectrum are not known, but studies withelectric lighting are available. For example,Brainard et al.54 and Thapan et al.53 foundeffects of the bluish part of the light spectrum(�max¼ 459–482 nm) on the suppression ofnocturnal melatonin. Gooley et al.75 foundthat short-duration (590min) exposure tolight from the greenish part of the lightspectrum (��¼ 555 nm;� 24 lx) was as effect-ive, if not more effective, than an equivalentphoton dose of 460-nm light (�2 lx) incausing a circadian phase shift.

Sahin and Figueiro76 found that a 48-minute exposure to short-wavelength (blue)light (40 lx, �max¼ 470 nm) and long-wave-length (red) light (40 lx, �max¼ 630 nm)equally affected human electroencephalogrammeasures indicating that acute melatoninsuppression is not needed to elicit an alertingeffect in humans. Interaction effects betweendifferent wavelengths and intensities are notfurther studied in the study of Sahin andFigueiro.76 However, Brainard et al.54 andThapan et al.53 performed a full actionspectra analysis for melatonin suppressionand both found a greater sensitivity of mela-tonin suppression to shorter wavelength light.The study of Gooley et al.75 suggested awavelength-dependent effect on circadianphase shift. This implies that multiple, if notall, parts of the light spectrum, at differentintensities play a role in triggering humanvisual and non-visual effects. Glazing is ableto filter certain parts of the radiation spec-trum, depending on the type. The questionwhether full-spectrum electric lighting canreplace daylight is not proven. According tosome studies77,78 there is evidence that full-spectrum electric lighting has comparableinfluence on, for example, cortisol andstress-related effects. However, the review ofMcColl and Veitch79 revealed little supportfor it.

Human vision under daylight conditions isnormally better than under electric lighting

due to the higher quantity (and often a bettercolour rendering index), enabling better visualperformance. Indirectly, vision can influencethe occurrence of depression. The sensitivityof the human alerting and cognitive responseto polychromatic light at levels as low as 40 lxis blue-shifted relative to the three-cone visualphotopic system.58 Daylight intensity is muchhigher than 40 lx during most of the day.Alerting affects general human physiologyand behaviour: The human body evolved andadapted in order to react to external triggers,with the 24-hour day/night cycle as one of themost important ones. People tend to preferthe high light levels of daylight, but do not(always) follow the natural variation in day-light.80 High radiation intensities are notalways desired since this radiation contains alot of energy which influences the heat load ofthe building. However, high light levels arebeneficial for groups such as older adults whorequire more light to perform well visually,but the opposite is true for people who sufferfrom migraine or have autism. There is apotential link between daylight and the inci-dence of migraine. Daylight openings withoutor with inadequate luminance screening orshading devices can lead to a large contrastbetween the daylight opening and the interiorwalls surrounding the opening. Cowlinget al.38 concluded that working in a buildingwith highly reflective windows and the pres-ence of blinds/curtains suggested the lowestfrequency of severe fatigue and eyestrain.Complaints of eyestrain may be related tothose of headache by a common neurologicalmechanism.37,80 In order to reduce triggersfor neurological attacks due to the high levelsof daylight and often occurring large contrast,controllable protection should be provided.

Aries et al.82 found that both view type andview quality had a significant influence onphysical and psychological discomfort. Inthis research, only the view itself was takeninto account, despite possible differences inview luminance. Surprisingly, nature views

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increased discomfort directly while view qual-ity negatively predicted discomfort (betterquality view was associated with lower dis-comfort). In relation to eyestrain, peopleshould have the possibility to focus on distantobjects, for example, by means of a viewoutside. At the same time, the minimumdistance is yet unknown. This may be relevantfor the further development of (virtual) win-dows and the basic design of buildings andthe surrounding landscape.

4.3. Practical implementations

Even though the majority of relevantinformation regarding daylight and healthdesign is not known or only very limited andmuch more research is necessary, some firstpractical implementations for building designare shown in Table 5. Nevertheless, these firstrecommendations can be followed by archi-tects and building physicists during the designof buildings and rounds of consultancy.

5. Conclusions

There is only limited statistically significantand well-documented scientific proof forthe link between daylight and its potential

health consequences, despite the omnipresentattention this supposed relation is receiving.This may sound rather counterintuitive.Nowadays, humans spend the majority oftime indoors, where they are often exposed topoor lighting, both in terms of quality andquantity. The amount of daylight people areexposed to indoors via windows is lower thanthe exposure outdoors. Further research isrequired to establish the nature of why somepeople prefer non-visual stimulating lightingand others do not. Also, dose–response curvesfor alertness, performance and mood todaylight need further investigation. In orderto ensure that the effects due to daylightexposure are not only applicable to peoplewith certain (health) conditions, future workshould focus on the effect of daylight on thehealth of the general population.

Fortunately, the search on specific healthkeywords produced more results, which weredivided into three categories according to theirassociation with daylight: ‘positive’, ‘negative’and ‘both positive and negative’. Nevertheless,the improvement regarding choice of words intitles, abstracts, search options or key wordscould help finding scientific evidence or know-ledge gaps. If the relation between daylightand health is fully understood and actually

Table 5 First practical implementations for daylight and health building design

Create daylight openings that can be opened to allow occasional exposure to the full radiation spectrum (includingultraviolet and infrared radiation)

Design buildings with floor plans that stimulate people to go outdoors, either via the ground floor or via (protected)verandas and balconies; independent of the weather conditions

Aim for rooms with relatively high daylight levels (E42000 lx on average vertically) and provide controllable sunlightand luminance protection (blinds, screens, etc.) on all daylight openings. The shading/protection gives people theopportunity to control and dose the entering light for the prevention and reduction of eyestrain, headaches,migraines, discomfort or disability glare, or photosensitive epilepsy, but maintains the option to have enoughdaylight quality and quantity for, for example, older eyes

Provide automated controls over blinds, luminance screens and shading that allow daylight access to the fullest.Especially in periods with sunrise and sunset during work time (winter time on the northern hemisphere), thedaylight opening should be uncovered to expose people to the change in photoperiod. However, users should beable to override the automated control at all times in order to meet personal comfort and health criteria (see also theprevious implementation)

Apply glazing that allows the transmittance of full-spectrum light in order to provide indoor lighting with all parts of thevisual spectrum represented so interaction effects between different wavelengths and intensities can naturally occurand are undisturbed





scientifically proven, future daylight design inbuildings will no longer be a mere recommen-dation, but an obligation.

Funding

This research received no specific grant fromany funding agency in the public, commercialor not-for-profit sectors.

Acknowledgements

The authors are very grateful to ProfessorEmeritus Anna Wirz-Justice PhD for hervaluable comments and helpful remarks onthe initial version of the review.

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