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Accident Analysis and Prevention 45S (2012) 62– 67

Contents lists available at SciVerse ScienceDirect

Accident Analysis and Prevention

jo ur n al hom ep a ge: www.elsev ier .com/ locate /aap

he effect of sleep restriction on snacking behaviour during a week ofimulated shiftwork

eorgina Heath ∗, Gregory D. Roach, Jillian Dorrian, Sally A. Ferguson, David Darwent, Charli Sargententre for Sleep Research, University of South Australia GPO Box 2471 Adelaide, South Australia 5001, Australia

r t i c l e i n f o

rticle history:eceived 1 May 2011eceived in revised form 4 August 2011ccepted 11 August 2011

eywords:leep restrictionircadian disruptionnackinghiftworkorced desynchrony

a b s t r a c t

Due to irregular working hours shiftworkers experience circadian disruption and sleep restriction. Thereis some evidence to indicate that these factors adversely affect health through changes in snackingbehaviour. The aim of this study was to investigate the impact of sleep restriction, prior wake and cir-cadian phase on snacking behaviour during a week of simulated shiftwork. Twenty-four healthy males(age: 22.0 ± 3.6 years, mean ± SD) lived in a sleep laboratory for 12 consecutive days. Participants wereassigned to one of two schedules: a moderate sleep restriction condition (n = 10) equivalent to a 6-h sleepopportunity per 24 h or a severe sleep restriction condition (n = 14) equivalent to a 4-h sleep opportunityper 24 h. In both conditions, sleep/wake episodes occurred 4 h later each day to simulate a rotating shift-work pattern. While living in the laboratory, participants were served three meals and were providedwith either five (moderate sleep restriction condition) or six (severe sleep restriction condition) snackopportunities daily. Snack choice was recorded at each opportunity and assigned to a category (sweet,savoury or healthy) based on the content of the snack. Data were analysed using a Generalised EstimatingEquations approach. Analyses show a significant effect of sleep restriction condition on overall and sweetsnack consumption. The odds of consuming a snack were significantly greater in the severe sleep restric-

tion condition (P < 0.05) compared to the moderate sleep restriction condition. In particular, the odds ofchoosing a sweet snack were significantly increased in the severe sleep restriction condition (P < 0.05).Shiftworkers who are severely sleep restricted may be at risk of obesity and related health disordersdue to elevated snack consumption and unhealthy snack choice. To further understand the impact ofsleep restriction on snacking behaviour, future studies should examine physiological, psychological andenvironmental motivators.

. Introduction

Traditionally there has been considerable focus on obesity as aajor public health concern. Recently there has been increasing

nterest as to how obesity affects performance in the workplace.besity has been associated with reduced productivity and higher

ates of absenteeism (Narbro et al., 1996) and in America the annualost of obesity on full time employment is estimated to be $73.1illion (Finkelstein et al., 2010). This is of significant concern to

ndustry and organisations, as obesity rates have risen dramati-

ally throughout the world over the past 15 years (World Healthrganisation, 2009).

∗ Corresponding author. Tel.: +61 8 8302 9946, fax: +61 8 8302 6623.E-mail addresses: Georgina.Heath@unisa.edu.au (G. Heath),

regory.Roach@unisa.edu.au (G.D. Roach), Jill.Dorrian@unisa.edu.au (J. Dorrian),ally.Ferguson@unisa.edu.au (S.A. Ferguson), David.Darwent@unisa.edu.auD. Darwent), Charli.Sargent@unisa.edu.au (C. Sargent).

001-4575/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.aap.2011.09.028

© 2011 Elsevier Ltd. All rights reserved.

According to the World Health Organisation, the rising ratesof obesity can be attributed to behavioural and lifestyle changesin society (World Health Organisation, 2009). Over the past twodecades there has been a shift to a “24-h society”. In turn, thedemand for industry, organisations and services to operate round-the-clock has lead to an increase in shiftwork in recent years(Australian Bureau of Statistics, 2010; U.S. Department of Labour,2005). Shiftwork is associated with lifestyle and behavioural fac-tors that may predispose shiftworkers to obesity. Therefore it isnot surprising that when compared to day workers shiftworkersare more overweight and obese (Atkinson et al., 2008) and sufferfrom numerous chronic health conditions related to obesity suchas cardiovascular disease (Boggild and Knutsson, 1999) and type 2diabetes (Kawachi et al., 1995).

Shiftworkers experience circadian disruption and sleep restric-tion and there is emerging evidence to suggest these two factors

may contribute to the increased incidence of obesity seen inshiftwork populations. Due to irregular working hours, shiftwork-ers sleep and eat at sub optimal circadian phases consequentlyaffecting the quality, quantity and distribution of food intake.

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ne of the first studies to examine eating behaviour in shift-orkers (Reinberg et al., 1979) found night shiftworkers consume

nacks throughout their shift and recent studies concur with thesendings (Waterhouse et al., 2003). Furthermore, studies suggesthiftworkers have poor dietary habits (Di Lorenzo et al., 2003;arda et al., 2005). A number of factors may contribute to the

nacking behaviour seen in shiftwork populations. Appetite hor-ones display a circadian rhythm (Cummings et al., 2001; Simon

t al., 1998) and altered metabolic responses to eating at nightuggest the body is not designed to consume and process fooduring this time (Atkinson et al., 2008). Therefore, shiftworkersay not be as hungry at night and thus prefer to snack rather

han consuming heavy meals (Waterhouse et al., 2003). Addi-ionally, the work place environment may contribute to snackhoice during night shift. Shiftworkers have limited access toanteen facilities when working night shift and often rely onending machines to obtain food (Stewart and Wahiqvist, 1985).lthough it is clear that shiftworkers consume snacks at inap-ropriate circadian phases, there are limited controlled studies

nvestigating the influence of circadian processes on snackingehaviour.

Along with circadian disruption, sleep restriction may alsoontribute to the high incidence of obesity seen in shiftworkers.hiftworkers report poor quality and quantity of sleep when com-ared to day workers (Akerstedt, 2003). This is of concern as aelationship between shorter sleep durations (<8 h) and obesityas been found in shiftwork populations (Moreno et al., 2006).aboratory studies have begun to investigate causal pathwayso help understand the link between sleep restriction, obesitynd related diseases. Participants in these studies have reportedn increase in appetite and cravings for calorie rich foods fol-owing sleep restriction (Spiegal et al., 2005). Additionally, theres evidence to suggest hormones involved in appetite regula-ion are disturbed as a result of sleep restriction. Studies havehown that levels of ghrelin (an appetite stimulating hormone)ncrease, while levels of leptin (an appetite reducing hormone)ecrease when individuals are deprived of sleep (Knutson et al.,007; Spiegal et al., 2005), indicating sleep restriction may result

n increased hunger. However, a laboratory study investigatingnergy intake following sleep restriction found that sleep restric-ion resulted in increased energy from snacks but not meals.n addition, there was no difference in leptin and ghrelin lev-ls when the two conditions were compared (Nedeltcheva et al.,009). This finding suggests snacking may be a result of pro-esses other than hunger (i.e. non homeostatic) and snackinghould be investigated separately to other eating episodes such aseals.Overall, snacking behaviour may be influenced by both circadian

nd homeostatic (i.e. prior wake/sleep) components. Additionallyhere is evidence to suggest sleep restriction impacts on snackingehaviour. Therefore a protocol which allows for examination of allhree factors is required. A forced desynchrony protocol extendsr shortens the waking day outside the range of entrainmentf the circadian pacemaker. This protocol desyncronises homeo-tatic and circadian processes and allows for the examination ofariables at all possible levels of prior wakefulness and circadianhase (see Kleitman and Kleitman, 1953 for further detail). Thusar, only one study (Waterhouse et al., 2004) has investigated theeparate influences of circadian and homeostatic processes on eat-ng behaviour, concluding food intake was governed more by the

ake period rather than circadian processes. However; partici-ants lived on a normal sleep:wake ratio of 1:2 (the equivalent

f an 8-h sleep opportunity per 24-h day). The current studyill expand on previous research by employing a forced desyn-

hrony protocol which will simulate a rotating shiftwork scheduleith two levels of sleep restriction (moderate and severe). As it

Prevention 45S (2012) 62– 67 63

is unclear if the level of sleep restriction affects snack consump-tion and snack choice, the current study aims to investigate theeffect of both moderate and severe sleep restriction on snack-ing behaviour. Furthermore, the study will examine the separateeffects of prior wake and circadian phase on snack consumptionand choice.

2. Materials and methods

2.1. Participants

Twenty-four healthy males participated in this study. Fourteenparticipants (age: 21.8 ± 3.8 years, mean ± SD) were randomly allo-cated to a severe sleep restriction condition and ten participants(age: 22.3 ± 3.5 years) were allocated to a moderate sleep restric-tion condition. Participants were of a healthy weight range (bodymass index: 22.2 ± 1.9 kg/m2), did not smoke and had normal sleep-ing patterns. Participants were excluded from the study if they weresuffering from a medical condition or using medication known toaffect sleep, if they had undertaken shiftwork or transmeridiantravel in the three months prior to the start of the study and ifthey consumed high levels of caffeine or alcohol. Ethics approvalfor this study was obtained from the University of South AustraliaHuman Research Ethics Committee. Written informed consent wasobtained from all participants and the research methods conformedto the guidelines established by the National Health and MedicalResearch Council of Australia.

2.2. Protocol

Participants spent 12 consecutive days in a purpose built, timeisolation laboratory at the Centre for Sleep Research. In both condi-tions, the first three days were 24 h in length (16 h of wake and an8-h sleep opportunity at night). These days allowed participants tofamiliarise themselves with the laboratory environment and trainon the performance tasks they were required to complete as partof a larger study. After the third night, participants in both con-ditions began the rotating shiftwork schedule which consisted ofseven 28-h ‘days’. The severe sleep restriction condition consistedof a 23.3-h wake episode and 4.7-h sleep opportunity (this is equiv-alent to a 4-h sleep opportunity per 24-h day). The moderate sleeprestriction condition consisted of a 21-h wake episode and 7-h sleepopportunity (this is equivalent to a 6-h sleep opportunity per 24-hday).

Throughout the study, participants were restricted from eatingad libitum. Participants were served standardised meals (i.e. break-fast, lunch, dinner) at the same elapsed time into each wake period(i.e. 0.5 h, 7.5 h, 15 h). Breakfast consisted of a choice of toast, fruit,yoghurt and cereal; lunch consisted of a choice of salad and coldmeats for sandwiches, soup and noodles; and dinner consisted ofa hot meal of rice or pasta served with meat and/or vegetables.Snack opportunities were also provided at the same elapsed timeinto each wake period (i.e. 2.5 h, 5 h, 10 h, 12.5 h, 17.5 h) in each con-dition. Participants in the severe sleep restriction condition wereprovided with an extra snack opportunity at 20 h. Water was madeavailable at all times. Participants chose and consumed their snackseparately to ensure their decision was not influenced by others.

As part of a larger study participants completed neurobehavioraltest batteries 1.5 h after waking and every 2.5 h thereafter duringwake periods. The results of these test batteries have been reportedelsewhere (Darwent et al., 2010; Sargent et al., 2010; Zhou et al.,

2010, 2011a,b). When not completing test batteries participantswere able undertake low intensity activities such as reading orwatching DVDs. To ensure compliance to the protocol participantswere monitored by a closed circuit television system.

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.3. Snacking

Snacks were assigned to one of three categories according to theontent of the snack: (1) Sweet (1 × 31.3 g muesli bar, 2 × 18 g sweetiscuits), (2) Savoury (1 × 19 g packet of potato chips, or 1 × 25 gacket of savoury crackers) and (3) Healthy (1 × piece of seasonalresh fruit, 1 × 40 g packet of dried fruit and nuts). Participants couldhoose one snack only from one of the three snack categories pernack opportunity; alternatively, they could choose not to snack.f participants chose to snack, they were given 30 min to consumehe snack. If the snack was not consumed within this time, it wasemoved by a member of the research team.

.4. Sleep

Sleep was monitored using a standard polysomnography mon-age which involved electrodes being attached to the scalp and facerior to each sleep opportunity. Sleep was scored in 30-s epochs inccordance with accepted criteria (Iber et al., 2007). Sleep efficiencyas calculated as percentage of time in bed spent asleep. Average

leep efficiency during the forced desynchrony phase of the pro-ocol for the moderate sleep restriction condition was 94.4 ± 2.4%nd average sleep efficiency for severe sleep restriction conditionas 95.9 ± 1.9%.

.5. Core body temperature

Core Body Temperature (CBT) was recorded continuously in 1-in epochs with indwelling rectal thermistors (Steriprobe 491B;

incinatti Sub-Zero Products, Cincinnati, Ohio). The thermistorsere connected to a Mini-Mitter datalogger (Bend, Oregon).

.6. Phase estimates

CBT data from six of the experimental days (2–7) were used toenerate circadian phase estimates for each participant. Follow-ng the cleaning of the raw CBT data, the data were de-masked forleep/wake differences and physical activity. A cosine equation washen fitted to the de-masked CBT data. The data were assigned aircadian phase estimate (i.e. 0–360 degrees) to each minute of thexperimental period of the study using a resultant cosine equationfor more details regarding this process see Darwent et al., 2010).

.7. Statistical analyses

Each snack opportunity was assigned a circadian phase (six binsith a width of 60 degrees, ∼4 h) and a level of prior wakefulness

five levels: 2.5 h, 5 h, 10 h, 12.5 h, 17.5 h of prior wakefulness). Thenack opportunity at 20 h of prior wakefulness (provided to the 4-hondition only) was excluded from the analysis to allow for equalomparisons between the conditions.

Logistic regression for a snack (yes/no) was conducted using aeneralised Estimating Equations (GEE) approach with exchange-ble correlation structure and robust standard errors accounting forlustering over subjects. Initial models included main and two-waynteraction terms. Where effects were significant within condition,lanned repeated contrasts (Wald statistics) were conducted forrior wake and core body temperature bins were compared to zero

egrees (circadian nadir) (see Fig. 1). Since interactions were notignificant, final models included main effects only. In order tourther investigate main effects of prior wake and phase, separate

odels were run for each condition.

Prevention 45S (2012) 62– 67

3. Results

3.1. Overall snack consumption

Significant main effects were found for overall snack consump-tion. There was a significant effect of condition, prior wake andcircadian phase (P < .05). There was a 40% reduction in the odds ofchoosing a snack in the moderate sleep restriction condition com-pared to the severe sleep restriction condition (95% CI = 0.36–0.95)(Table 1). Fewer snacks were consumed following a meal in bothconditions (Fig. 1). All circadian phases were significantly differentto the circadian nadir in the severe sleep restriction condition suchthat there were higher frequencies of snack consumption duringthe biological day compared to the biological night (Fig. 1). Therewere no significant differences between the phases in the moderatesleep restriction condition.

3.2. Sweet snack consumption

Significant main effects were found for the consumption ofsweet snacks. A significant main effect was found for condition(P < .05), and prior wake was approaching significance (P = 0.06)(Table 1). Separate models were then run for each condition. Whereeffects were significant within condition, Wald comparisons wereconducted to test the effect of prior wake (planned contrasts) andcircadian phase (compared to bin zero) on sweet snack consump-tion for each condition. The odds of choosing a sweet snack werereduced during the snack opportunity following lunch and dinner.Overall, the odds of choosing a sweet snack increased across theday in the severe sleep restriction condition. No significant effectswere found for circadian phase on sweet snack consumption.

3.3. Savoury snack consumption

A significant effect was found for prior wake on savoury snackconsumption (P < 0.05) (Table 1). Wald comparison showed thatthere were reduced odds of consuming a savoury snack at 17.5 hof prior wakefulness compared to 12 h of prior wakefulness. Nosignificant effects were found for condition or circadian phase onsavoury snack consumption.

3.4. Healthy snack consumption

No significant effects were found for condition, prior wake orcircadian phase on healthy snack consumption (Table 1).

4. Discussion

The current study investigated the influence of sleep restric-tion (severe and moderate) on snacking behaviour. Furthermore,the design of the study allowed for separate examination of home-ostatic (prior sleep/wake) and circadian processes on snackingbehaviour. Results suggest that individuals who are severely sleeprestricted to the equivalent of 4 h sleep opportunity per 24 h hadgreater odds of consuming a snack when compared to individu-als restricted to the equivalent of 6 h of sleep per 24 h. Moreover,when snack choice was examined, odds of choosing a sweet snackwere increased when sleep was severely restricted. No significantdifferences were found between the two conditions for the con-sumption of savoury or healthy snacks. The lowest frequencies ofsnack consumption were around the circadian nadir.

The above findings concur with results from a previous labora-

tory study in which participants who were restricted to 5.5 h ofsleep per night consumed more energy from snacks than whenthey were provided with an 8.5 h sleep opportunity per night(Nedeltcheva et al., 2009). The study concluded that the increased

G. Heath et al. / Accident Analysis and Prevention 45S (2012) 62– 67 65

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ig. 1. Frequency of overall, sweet, savoury and healthy snacks by condition. Prior wlotted) were compared to zero degrees (circadian nadir). *Represents significant diP < 0.05) for moderate sleep restriction condition.

onsumption of snacks was not due to increased energy needs.n contrast to the present study, Nedeltcheva et al. (2009) foundhat the increase in snacking was more likely to occur at nightather than during the day. An advantage of the current study washat the forced desynchony protocol allowed for examination ofomeostatic and circadian processes separately. When these pro-esses were separated, we found that snacks were consumed morerequently during the biological day compared to the biologicalight. These contrasting findings demonstrate the need to sepa-ate homoeostatic and circadian processes to ensure results are not

onfounded.

Interestingly, shiftworkers consume more snacks at work whenompared to day workers (Waterhouse et al., 2003). This behaviouray be due to the severe sleep restriction often reported by night

ata were analysed using planned repeated contrasts. Circadian phase data (doublece (P < 0.05) for severe sleep restriction condition. ˆRepresents significant difference

shift workers. Indeed, nurses working night shift have reportedthey consume sweet snacks as they perceive it will help keep themawake (Persson and Martensson, 2006). Furthermore environmen-tal factors may influence the snacking behaviour of shiftworkers.There are limited food options available at night and shiftworkersmay not have the time to prepare healthy meals before their shift.Furthermore the investigation of reasons behind snack consump-tion and choice in shiftwork populations will demonstrate the roleenvironmental factors may play.

In the current study only limited snack choices were offered

to participants, however, even with the narrow range of choicewe found significantly increased odds of choosing snacks fromthe sweet category when sleep was severely restricted. Further-more, these findings concur with previous research suggesting the

66 G. Heath et al. / Accident Analysis and Prevention 45S (2012) 62– 67

Table 1Main effects of condition (moderate/severe), prior wake and circadian phase for overall, sweet, savoury and healthy snack consumption.

Dependent variable Independent variable Odds ratio (OR) St error of OR Z P 95% CI lower 95% CI upper

Snacking (overall) Condition 0.58 0.12 −2.17 0.03 0.36 0.95Prior Wake 0.82 0.07 −2.29 0.02 0.69 0.97Circadian Phase 1.25 0.11 2.59 0.01 1.06 1.48

Sweet Snack Condition 0.54 0.12 −2.81 0.01 0.35 0.83Prior Wake 1.13 0.07 1.86 0.06 0.99 1.28Circadian Phase 1.07 0.06 0.18 0.18 0.97 1.19

Savoury Snack Condition 0.87 0.15 −0.78 0.44 0.62 1.22Prior Wake 0.86 0.06 −2.37 0.02 0.75 0.97Circadian Phase 0.99 0.03 −0.05 0.96 0.93 1.07

Healthy Snack Condition 1.16 0.26 0.65 0.52 0.75 1.79Prior Wake 0.94 0.05 −1.16 0.25 0.84 1.05

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t error, standard error. CI, confidence interval.

nergy from snack consumption in sleep restricted individuals wasore likely to come from carbohydrates (Nedeltcheva et al., 2009).lthough macronutrient content was not analysed in the currenttudy, the snack choices in the sweet category had a high per-entage of carbohydrate. Interestingly, carbohydrate cravings aressociated with depressed mood (Lieberman et al., 1986) and bothhiftworkers and severely sleep deprived individuals are reportedo experience mood disturbances (Dinges et al., 1997; Gordon et al.,986). This further demonstrates the need for future research to

nvestigate psychological motivations behind snack choice as mooday mediate the relationship between sleep restriction and snack

hoice.The forced desynchrony protocol, which allowed for separate

nalysis of homeostatic and sleep related processes on snackingehaviour, has built on previous sleep restriction studies. How-ver, it is important to note there were some limitations of theurrent study which may reduce external validity and general-sation to shiftworkers. For example, to ensure a homogenousample, the current study only included healthy young males;uture studies should investigate snacking behaviour in femalesnd older age groups during sleep restriction. Although the cur-ent protocol allowed for examination of snacking behaviour atll possible levels of prior wake and at all circadian phases, theotating schedule applied in this study does not correspond with

typical rotating shift schedule. Furthermore, as the protocol waselatively short, the long-term effects of sleep restriction on snack-ng behaviour cannot be determined. Nonetheless, our findingsuggest that when individuals are severely sleep restricted they areore likely to snack on sweet foods. As a result, individuals who

re severely sleep restricted may be at risk of weight gain due tooor food choices. This finding has implications for shiftwork pop-lations as they often report severe sleep restriction (Akerstedt,003) and compared to day workers, shiftworkers are more over-eight and obese (Atkinson et al., 2008). Future research should

ocus on understanding the mechanisms involved in unhealthynack choice and consumption when sleep is severely restrictedncluding: psychological (i.e. mood, perception of alertness) phys-ological (i.e. appetite as a result of hormonal disruption), andnvironmental factors (i.e. time constraints, facilities available).

greater understanding of shiftworkers’ snack consumption andhoice will allow for the preparation of health promotion strate-ies, which will in turn be beneficial for both individuals andrganisations.

cknowledgements

The authors gratefully acknowledge the financial support of theustralian Research Council and the Centre for Metabolic Fitness.

1.10 0.27 0.97 1.10

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