The purpose of sleep is not fully understood, but it is thought to be an important period of restoration that is aided by hormonal changes. Research shows that adults who attain less than 7 hours of sleep a night have a significantly higher risk of becoming obese compared to those who sleep over 8 hours. This association may come about because sleep duration can alter appetite hormones such as leptin and ghrelin, and this in turn may alter the subsequent energy intake. Hormonal changes are a known physiological phenomenon resulting from sleep deprivation and it is possible that this homeostatic disruption could result in metabolic disturbances. Because of this, researchers are interested in the effects of sleep deprivation on appetite and energy intake, in order to elucidate mechanistically the reason that obesity is associated with lack of sleep.
For example, researchers1 have investigated the effects of sleep deprivation on the energy balance of 15 men and 15 women aged 30 to 49 years. The subjects regular sleep duration was 7 to 9 hours per day and the mean body mass index was between 22 and 26kg/m2. Subjects were randomly allocated to receive either 4 or 9 hours sleep for 5 days before crossing over to the other treatment. Food intake was measured on day 5 of the treatment periods and energy expenditure was measured using the doubly labelled water method throughout each period. Those subjects having 4 hours sleep a night consumed more energy on day 5 of the study compared to those with 9 hours sleep. This increased energy intake was mainly in the form of saturated fat. There was no statistical difference in energy expenditure or resting metabolic rate between different sleep durations.
The ad libitum energy intakes of those who are exposed to sleep deprivation are greater than those who are provided with habitual sleep. In this study, the increase in energy intake was not accompanied by an increase in the thermic effect of food, suggesting that the extra calories could lead to increased adiposity. The authors reported the increase in energy intake caused by sleep deprivation to be around 300kcals/d, but importantly, the subjects reported no increased appetite or feeling of hunger. Taken as a whole, these results suggest that 4 nights of sleep deprivation are enough to increase energy consumption in young, healthy normal weight individuals. Although it is unclear if this would lead to weight gain this study may explain the association between short-sleep duration and obesity. This provides more evidence that sleep deprivation disrupts normal metabolism.
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