Doctors and scientists have known for decades that insufficient sleep affects the body's hormone levels and ability to regulate and metabolize glucose. That means if you're excessively sleepy, you could be at higher risk for weight gain (see "Sleep and Obesity") and even type 2 diabetes.
Researchers have tested the relationship between sleep and hormones extensively. In one study, healthy adults were asked to sleep only four hours a night for six nights. After this period of sleep restriction, the subjects' glucose tolerance (their ability to break down glucose) was 40 percent lower on average—reaching levels that are typical of older adults at risk for diabetes, which is characterized by high glucose levels due to insufficient insulin. Additionally, when the sleep-deprived subjects were fed a high-carbohydrate breakfast, their glucose levels stayed significantly higher than when they were well rested—providing further evidence that their bodies were not processing glucose as well.
Why would lack of sleep affect hormones and glucose metabolism? Part of the answer may lie in slow wave sleep. When a person enters slow wave, or deep sleep, nervous system activity goes down, the brain uses less glucose, and other changes occur such as an increase in growth hormone and a decrease in the activating hormone cortisol. For this reason, a sufficient amount of deep sleep is thought to be very important to the regulation of glucose in the body.
Researchers have seen this in action by manipulating people's sleep stages and looking at the effect it has on their glucose levels. In one experiment, scientists disrupted people's sleep just enough to keep them from entering deep sleep (but not enough to fully wake them). After these nights of deep-sleep deprivation, the subjects' insulin sensitivity and glucose tolerance went down by 25 percent.
Doctors believe this is one of the reasons for the relationship between insufficient sleep, weight, and diabetes risk. Short sleep and poor sleep quality up the risk of a person being overweight and developing diabetes, and the disruption in glucose may be one of the factors at the center of this problem.