Stress can come in many different forms and the perception of stress can be highly subjective. For this reason, what appears to be a stressor for one individual can be discounted as a stressor for another. For example, common daily stressors such as visiting the doctor, traveling to work and moving house, are not perceived as stressors to some. However, a number of stressors are known to almost universally result in a stress response in all animals. Of these universal stressors, exercise and sleep deprivation are perhaps the most exemplified in the literature and the most widely established for their detrimental effects on hormonal and nervous system activity. However, while evidence suggests that organisms adapt to the stresses of exercise such that greater and greater tolerance is achieved through training, there is no such evidence that sleep deprivation can be overcome through any other mechanisms that sleeping for longer at a future date. This makes sleep deprivation a devastating stressor if accumulation of the sleep debt continues chronically.
Sleep deprivation is known to cause a number of negative effects such as a decrease in reaction times, delayed thinking, tiredness, irritability and lethargy. However, as with all stressors, sleep deprivation also results in changes to hormonal activity, such that cortisol and adrenaline are released. Chronic sleep deprivation has been investigated with regards to obesity, and current evidence suggests that sleep deprivation is associated with weight gain. In other words those that tend to sleep less than their required sleep duration tend to be at an increased risk of weight gain and obesity. The cause and effect of the association are not fully understood, and it could be for example that as body weight increases sleep deprivation is more likely. Sleep apnoea is a common condition in obese individuals and results from obstruction to the airway during sleep, something that is more likely with obesity. This interruption to breathing during sleep. It is therefore difficult to emphatically state that sleep deprivation is the cause of obesity.
However, it is known that sleep deprivation is also associated with the stress response (including the release of cortisol) and associated with the development of insulin resistance. It is also known that cortisol release can be a cause of insulin resistance and weight gain, as exemplified by the synthetic cortisol drug prednisolone having the common side effect of both insulin resistance and weight gain. There is therefore good reason to suggest that chronic sleep deprivation could increase the risk of weight gain through a decrease in the insulin sensitivity of tissues. Sleep deprivation has been shown to interfere with glucose homeostasis in healthy young individuals under laboratory conditions, and this has been shown to be accompanied by decreases in the plasma levels of leptin, and reductions in the anorexigenic hormones that reduce appetite. These changes suggest that sleep deprivation increases appetite and decreases metabolic rate, two factors that could result in weight gain over the long term.
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