The Physiology of Sleep Deprivation
Inadequate Rest can Have Profound Metabolic Consequences
Dec 5, 2008
Over the past 50 years, average sleep duration for Americans has decreased by 1.5-2 hours per night. Shortened sleep seems to be a trademark of modern society. (National Sleep Foundation. Sleep in America Poll. 2001-2002)
Firemen, physicians, law enforcement personnel, and people who work rotating shifts are among those groups whose sleep patterns are significantly disrupted. But many people routinely spend only five or six hours in bed each night.
However, far from being a harmless virtue of efficient and dedicated workers, sleep deprivation affects hormones, metabolism, and immune function in ways that can threaten one’s health.
Physiologic Effects of Sleep Deprivation
Sleep curtailment affects two major pathways of hormone production:
- Pituitary-hypothalamic axis: The pituitary—rightly called the “master” gland, because it controls the release of hormones from the peripheral endocrine glands—is itself partly influenced by the release of factors from the hypothalamus. Changes in sleep patterns profoundly affect the levels of these hypothalamic factors and, hence, pituitary hormones. For example, levels of growth hormone (GH) releasing factor increase during sleep, while those for corticotropin decrease.
- Autonomic nervous system: Usually, sympathetic tone decreases during sleep, while parasympathetic tone increases. Therefore, sleep loss leads to generally higher sympathetic and lower parasympathetic tone. Most endocrine organs are exquisitely sensitive to changes in autonomic tone. Recent studies have shown that chronic curtailment of sleep, as is the case with many American workers, may contribute to the development of conditions that are major health issues in our society.
Obesity and Sleep Deprivation
Sleep and appetite are closely intertwined. Curtailment of sleep leads to a marked decrease in levels of leptin (a hormone that inhibits appetite) and an increase in levels of grehlin (stimulates appetite). The resulting hormonal imbalance leads to an increase in appetite that is disproportionate to the caloric demands of the extended hours of wakefulness. (Spiegel K, et al. Sleep curtailment in healthy young men is associated with decreased leptin levels: elevated ghrelin levels and increased hunger and appetite. Ann Intern Med. 2004;141:846-850)
Thus, long-term sleep curtailment leads to metabolic signals that mimic a state of famine in the face of plentiful opportunities for eating.
Metabolic Syndrome, Diabetes and Sleep Deprivation
Cortisol and growth hormone are both “counter-regulatory” hormones; they oppose the action of insulin and serve to drive blood glucose upward. Sleep deprivation increases the levels of both of these hormones.
In addition, sleep curtailment seems to impair the glucose-lowering effects of insulin; blood glucose levels are higher than normal after eating despite normal to slightly elevated levels of insulin in the bloodstream. (Spiegel K, et al. Impact of sleep debt on metabolic and endocrine function. Lancet. 1999;354:1435-1439)
Finally, pancreatic insulin secretion is altered by changes in autonomic nervous tone elicited by sleep curtailment. The result of these—and as yet unidentified—physiologic aberrations is a marked tendency toward the development of metabolic syndrome and overt diabetes.
Thyroid Dysfunction and Sleep Deprivation
Sleep deprivation evokes striking decreases in the production of thyroid stimulating hormone (TSH). In an otherwise normal individual, low TSH leads to a hypofunctional thyroid, with potentially significant effects on growth, glucose and fat metabolism, immune function, and cognition.
Clearly, sleep deprivation—a condition that is not only common in our society, but actively encouraged—may be intimately tied to many chronic diseases.
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