The purpose of sleep in humans and animals is not fully understood. However, during sleep certain restorative processes are thought to occur, and hormonal cascades are hypothesised to be part of this process. Sleep is therefore necessary for normal physiological function and deprivation of sleep is known to alter hormone levels and in particular raise plasma levels of cortisol. Short-term sleep deprivation may have no detrimental health effects as compensatory sleep can restore normal function and rebalance hormone levels. However, chronic sleep deprivation can cause serious ill heath and eventually death. Plasma cortisol levels rise during sleep deprivation because lack of sleep is a form of stress. The rise in plasma cortisol is thought to have implications for maintaining the correct body weight. This is because cortisol interferes with normal metabolic regulation and alters blood sugar, insulin and leptin levels, and as such affects the storage and utilisation of energy.
Because of the possibly implications for obesity, researchers1 have investigated the effects of sleep deprivation on body weight in adults by measuring sleep over the course of 4 to 5 nights in women and men, respectively. Subjects were between the age of 67 and 96 years old. The researchers controlled for known obesity risk factors and measured a sub-group of subjects for sleep apnoea in order to remove any bias this might have with the results. Following the statistical adjustments, the authors reported that sleep duration of less than 5 hours per night was associated with a body mass index that was on average 2.5 and 1.8 kg/m2 greater in men and women, respectively. This translated into a 3.7 fold increase in the odds of suffering from obesity for men, and a 2.3 fold increased odds of suffering from obesity in women, if under 5 hours sleep was obtained per night.
These results support previous studies, particularly those involving children, that demonstrate an association between obesity and sleep deprivation. Because it could be argued that obesity caused sleep deprivation though sleep apnoea, the authors were careful to control for this factor. However, even so, it is still not possible to ascribe causes and effect from these results. Interestingly, the researchers reported that short sleep duration was associated with abdominal obesity as well as an increased percentage of total body fat. Abdominal obesity is a known result of the presence of metabolic syndrome and is a risk factor for cardiovascular disease. High plasma cortisol levels, that may develop from chronic sleep shortage, might modify the compartmentalisation of fat from subcutaneous to abdominal fat. This may be due to the development of insulin resistance through a cortisol induced increase plasma glucose level.
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