Sleep, a process as natural to most humans as breathing, often remains unappreciated until the painful symptoms of its absence—the lethargy, impaired cognition, and increased appetite known to many—manifest themselves. Sleep’s importance was bluntly unveiled in an 1894 study that deprived dogs of sleep until they died. “In contrast to dogs dead from starvation, those killed from loss of sleep showed that the cerebrum, of all organs, was the most seriously and extensively altered” wrote the authors.1 If the authors could have sampled hormones, though, they would have uncovered much more than cerebral degeneration. As researchers now know, acute sleep deprivation disrupts the body’s normal hormonal profile.
Hormones are crucial molecules that allow for communication between bodily cells and also for systemic changes within an organism. The sight of a charging bear unleashes a signaling cascade that travels from the brain to the kidney’s adrenal glands, where the hormones adrenaline and cortisol trigger the “flight or fight” response. The hormonal response allows the body to adapt to changing environmental stimuli, a necessity for survival. Today, both over and underproduction of hormones result in serious medical disorders: without the hormone insulin, blood glucose is not metabolized and diabetes occurs; without the satiety hormone leptin, uncontrollable eating and severe obesity ensue.2
Crucially, hormones flux up and down in patterns partially controlled by the body’s circadian rhythm. For example, the stress hormone cortisol is lowest at the beginning of the night, allowing for sleep. Conversely, leptin levels are lowest upon awakening, allowing one to feel hunger, a feeling dependent upon another hormone, ghrelin.2 Proper hormone levels play a critical role in maintaining homeostasis and well-being, a fact that makes the current hormonal studies done by the University of Chicago’s Sleep, Metabolism, and Health Center (SMAHC) important and disconcerting.
Unlike the 1894 study, absolute sleep deprivation and death are not enforced by SMAHC. Instead, only four hours of sleep are allotted to healthy subjects for a certain number of nights. Throughout this “deprivation session,” hormones, blood glucose, appetite, and alertness are monitored and then compared to readings taken when subjects were well rested. The resultant hormonal changes are striking: a 1999 SMACH study showed that insulin’s response to glucose decreased by 30% when subjects were deprived of sleep. Furthermore, the efficacy of the body’s glucose uptake (“glucose clearance”) also decreased by 30%.3 These two metabolic changes are hallmarks of type II diabetes. However, Dr. Kristen Knutson, a researcher at SMAHC, cautions that it is “tough to determine whether sleep is causal in the diabetes epidemic. The laboratory studies of sleep restriction certainly suggest that insufficient sleep can impair glucose control in the short term.”4
SMAHC’s 1999 study also showed that sleep deprived subjects had elevated evening cortisol levels that decreased six times slower than the normal declination preceding sleep. Along with changes in glucose clearance, this hormonal shift is comparable to the effect aging has on hormonal profiles.3 SMAHC has made the comparison between sleep deprivation and aging elsewhere: in a 2011 study, testosterone levels in men decreased 15% when they were deprived of sleep, a change analogous to aging between seven and fifteen years.5 “Thus, this raises the question of whether sleep loss plays a role in some biomarkers of aging…” writes Dr. Knutson.4 So does sleep deprivation predict how we’ll feel twenty years down the road? Not exactly: Dr. Knutson notes that hormone changes in sleep studies occur quickly and reversibly, unlike the slow process of aging—a statement that brings no small relief to those who recall the soporific nausea of their last final’s week.
As college students can readily attest, sleep deprivation engenders not only lethargy but also a ravenous hunger for unhealthy foods. While some attribute poor eating choices to impaired self-control, research at SMAHC has shown an underlying hormonal shift partially responsible for the unhealthy cravings of the tired. In a 2004 study, sleep deprived subjects had an 18% decrease in the satiety hormone leptin and a 28% increase in the hunger hormone ghrelin.6 Combined with data showing poorer metabolic control following sleep deprivation, this experiment provides chemical evidence to support the surprising finding that shortened sleep may play a role in the obesity epidemic. In one such study, researchers analyzed the sleep habits and body mass index (BMI) of twins to control for genetic differences. Intriguingly, twins that slept less than seven hours per night had a significantly higher BMI than their longer-sleeping counterparts, while twins that slept equal amounts had no significant BMI differences.7 A separate study on European adolescents found a significant correlation between decreased sleep and increased BMI.8
Other researchers are skeptical of the correlation between shorter sleep, obesity, and metabolic disorders. Dr. Jim Horne, a Loughborough University researcher opposed to the more sweeping claims made by SMAHC, writes that “obesity is only significantly associated with habitually short sleep when it is <5 h and >9 h per day.” While Horne does not dispute that hormones were perturbed by sleep deprivation, he contends that it is “unclear to what extent these findings can be generalized even to 5 h habitual sleepers.” Horne argues that fear over society’s sleep deprivation is incorrectly inflated and that sleep research must be rigorously controlled before hasty, unsupported claims are made.9
Even so, research undertaken by SMAHC has shed new light on the interplay of sleep and hormones. Severe sleep deprivation has been shown to shift many of the body’s hormones, including those responsible for proper glucose metabolism, appetite regulation, and stress. This shift might have once been beneficial, a past adaptation that allowed for increased food intake and energy when times were tough. Nowadays, warns University of Chicago’s Dr. Plamen Penev, the shift “can be maladaptive in the context of a modern environment that allows for many to overeat while maintaining a sedentary lifestyle without sufficient sleep.”10
- Manaceine, M. 1895. “Experimental Observations upon the Influence of Absolute Insomnia” American Medico-Surgeon Bulletin. Vol. 8 Accessed July 15, 2012.
- Van Cauter, E., Knutson, K., Leproult, R., and Spiegel, K. 2005. “The Impact of Sleep Deprivation on Hormones and Metabolism.” Medscape Neurology 2005;7(1). Accessed August 26th, 2012.
- Spiegel, K., Leproult, R., and Van Cauter, E. 1999. “Impact of Sleep debt on Metabolic and Endocrine Function.” The Lancet 354:1435-1439. Accessed July 15, 2012. doi: 10.1016/S0140-6736(99)01376-8
- Knutson, Kirsten. Interview with the Author (email). July 15, 2012.
- Leproult, R. and Van Cauter, E. 2011. “Effect of 1 Week of Sleep Restriction on Testosterone Levels in Young Healthy Men.” JAMA 305(21):2173-2174. Accessed July 15, 2012. doi:10.1001/jama.2011.710
- Spiegel, K., Tasali, E., Penev, P., and Van Cauter, E. 2004. “Brief Communication: Sleep Curtailment in Healthy Young Men Is Associated with Decreased Leptin Levels, Elevated Ghrelin Levels, and Increased Hunger and Appetite.” Annals of International Medicine 141(11):846-850. Accessed July 15, 2012.
- Watson, N., Buchwald, D., Vitiello, M., Noonan, and C., Goldberg, J. 2010. “A Twin Study of Sleep Duration and Body Mass Index.” Journal of Clinical Sleep Medicine 6(1): 11–17. Accessed July 15, 20102.
- Garaulet, et al. 2011. “Short sleep duration is associated with increased obesity markers in European adolescents: effect of physical activity and dietary habits.” International Journal of Obesity 35(10):1308-17. Accessed July 15, 2012. doi: 10.1038/ijo.2011.149
- Horne, J. 2010. “The end of sleep: ‘Sleep debt’ versus biological adaptation of human sleep to waking needs.” Biological Psychology 87(1): 1-14. Accessed July 15, 2012. doi: 10.1016/j.biopsycho.2010.10.004
- Penev, P. 2012. “Update on Energy Homeostasis and Insufficient Sleep.” JCEM 97(6): 1792-1801. Accessed July 15, 2012. doi: 10.1210/jc.2012-1067
- Image credit (Creative Commons): “Shanghai killer whale.” 2012. “Chronic fatigue syndrome.” Wikimedia Commons. Accessed October 18, 2012.