Jean-Philippe Chaput1, Isabelle J. Dionne2,3 and Angelo Tremblay1,*
1Division of Kinesiology, Department of Social and Preventive Medicine,
Faculty of Medicine, Laval University, Quebec City, Quebec, Canada, G1K 7P4.

2Research Centre on Aging, Geriatric Institute of the University of Sherbrooke,
1036 Belvédère Sud, Sherbrooke, Quebec, Canada, J1H 4C4.

3Faculty of Physical Education and Sports, 2500 boul. de l'Université,
University of Sherbrooke, Sherbrooke, Quebec, Canada, J1K 2R1.

Introduction
Obesity is one of the most important public health problems today, and although much has been learned regarding the regulation of body weight, the prevalence of obesity continues to rise. Excessive caloric consumption and/or lack of physical activity seem to account only for a fraction of this increase in body weight. Current treatments for obesity have been largely unsuccessful in maintaining long-term weight loss, suggesting the need for new insight into the mechanisms that result into altered metabolism and behavior and may lead to obesity. Parallel to an increase in body weight, there has been a reduction in sleep times (1). Indeed, lack of sleep has become a widespread habit driven by the demands and opportunities of our modern "24-hour" turbulent lifestyle. Nearly one-third of adults report sleeping less than 6 hours per night, leading some to suggest that we live in a sleep-deprived society (2). Not surprisingly, reports of fatigue and tiredness are more frequent today than a few decades ago (3).

Cross-sectional (4-6) and longitudinal (7,8) studies in adults have repeatedly found an association between reduced sleep and increased weight. Similar findings have been observed in children, suggesting that short sleep duration correlates with an increased risk of being overweight or obese (9-11). Furthermore, recent research data from our laboratory showed that the increase in body weight associated with short sleep duration is preferentially deposited in the abdominal region (12). Since there is an increased recognition that abdominal obesity is the most prevalent feature of the metabolic syndrome, i.e. a cluster of diabetogenic, atherogenic, prothrombotic and proinflammatory metabolic abnormalities (13), these results add novel insights to the negative impact of short sleep duration in children.

Of course, the obvious question is: "How short sleep duration is related to obesity?" Ultimately, in order to cause weight gain, short sleeping hours must increase caloric intake and/or decrease energy expenditure. In this respect, one potential explanation may be that when we sleep less, we simply have more time and/or more opportunities to eat. In addition, restricted sleep may lead to daytime fatigue and perhaps reduced physical activity. However, much attention has recently been focused on the responses of ghrelin, leptin and orexin to sleep restriction (4,14,15). Alterations in these hormone levels or patterns of secretion may affect hunger and appetite, increasing the risk of overeating and consequently weight gain. They may also affect thermogenesis from activities other than exercise. Non-exercise thermogenesis (such as seen in fidgeting and posture changes) is a variable component of energy expenditure and has been reported to account for differential weight gain in rats (16). In addition, sleep loss results in changes in levels of several other hormones such as cortisol and growth hormone (17). However, the precise mechanisms by which the brain modulates hormone release with sleep loss is unknown, but one possibility is increased sympathetic nervous system activity (18,19).
Thus, short sleep duration may be perceived as a stress factor having a destabilizing impact on body homeostasis. In this regard, lack of sleep is related to a greater risk of being overweight probably because it prevents the recovery of a hormonal profile facilitating appetite control. In this context, fat gain contributes to at least a partial compensation since adipose tissue hypertrophy favors an increase in plasma leptin concentration.

Does short sleep duration favor overweight or obesity in older people?
Although a growing body of evidence supports an association between short sleep duration and the risk for obesity, the literature is nonexistent regarding the documentation of this issue in older people. In a 13-year prospective study conducted in almost 5,000 young adults, Hasler et al. (7) showed that the associations between sleep duration and obesity diminished after age 34 years, albeit the age of participants did not exceed 40 years. However, we recently published results showing that short sleep duration does not predict an increased risk of being overweight or obese in older women (20). To our knowledge, it is the first study showing data about the sleep-body weight connection in the elderly. We were surprised by the results of our study since they are not concordant with the relevant literature in this field of investigation. Although it is difficult to explain the absence of relationship between sleep and body composition in older people as observed in younger age groups, some arguments might be considered. Indeed, it is possible to argue that the decrease in the sensitivity to leptin and ghrelin with age (21,22) might explain in part the observed discrepancy. Moreover, recent data indicate that anorexigenic signals (e.g., leptin) prevail over orexigenic signals (e.g., ghrelin) in healthy elderly, contributing to prolonged satiety and inhibition of hunger, which may lead to calorie deficit (22). Nevertheless, further studies are highly needed in order to confirm these results and to further investigate the sleep?body fat associations in older people.

Conclusions
Our modern world, with its hectic lifestyle, has favored the dominance of factors promoting positive energy balance in which short sleep duration is potentially associated with alterations in the hormonal profile that impair the regulation of energy balance. Given the compelling body of work supporting the associations between short sleep duration and obesity in children and adults, sleep time must now be considered as a new and potentially important determinant of obesity in the current way of living. Since preventing obesity is important, a pragmatic approach adding sleep hygiene advice to encouragement towards a healthy diet and physical activity may help tackle the obesity pandemic. However, there is an urgent need for studies aiming to investigate the sleep-body weight connection in the elderly. By doing so, this will help unravel this issue in this population and to establish its clinical significance.

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1,* Address for correspondence: Angelo Tremblay, Ph.D.
Division of Kinesiology (PEPS)
Laval University
Quebec City, Quebec, Canada
G1K 7P4
Tel: (418) 656-7294
Fax: (418) 656-3044
E-mail: angelo.tremblay@kin.msp.ulaval.ca