Not all of the studies found such drastic differences in hormone levels. But many have also documented study subjects eating more and/or more often in the lab after they have had consecutive nights of partial sleep deprivation. One study tested women, reducing their nightly sleep from seven to four hours over the course of four nights. During the sleep deprivation phase, women ate an average of about 400 more calories daily than they had at the beginning of the session—and even gained weight over the course of the short study.
As Shlisky and her colleagues point out in their paper, people who are awake longer simply have more opportunities to eat. "Partial sleep deprivation may increase the risk of overeating in the evening due to low circulating leptin levels and additional time spent awake," the researchers noted, adding that "evening and late-night hours are when overeating of less-healthful foods is most likely to occur." Furthermore, additional research that shows "impulse control and delayed gratification are lowered with sleep deprivation, perhaps making sleep-deprived individuals more vulnerable to hedonic eating," rather than resort to healthful foods to sate hunger.
Stressed and sleepy
Do these extra waking hours also help us get in more physical activity? As with eating, it might seem reasonable to think that being awake longer would lead to more beneficial activity, resulting in more energy expended. Not necessarily, according to the researchers. One study found that after just two nights of being allowed only four hours asleep, subjects had "significantly lower activity" than those subjects allowed eight hours rest. Although lab study results on exercise levels after sleep deprivation have been mixed, people generally report feeling more lethargic and less capable of getting the recommended moderate- to high-intensity exercise. So, although one might spend an extra two to four hours prone in bed, during the 16 to 17 hours of wakefulness a well-rested person would be more likely to meet exercise recommendations and improve his or her metabolism.
Sleep deprivation can also lead to muscle loss and fat gain. With too little sleep, the body is also more likely to produce the stress-response hormone cortisol. After sleep deprivation, subjects in several studies had higher levels of cortisol later in the day, a time when it should be tapering off to prepare the body for rest. Heightened cortisol prompts the body to store more fat and be more inclined to use other soft tissue, such as muscle, as energy, which means that sleep-deprived dieters lose more muscle and gain more fat than do those who are well rested. One study found that after two weeks of minor calorie restriction (10 percent less than their daily energy expenditure), subjects who were getting 5.5 hours in bed a night lost just 0.6 kilogram of fat but 2.4 kilograms of other tissue, such as muscle; subjects who got 8.5 hours slumber each night lost 1.4 kilograms of fat and 1.5 kilograms of other tissue. "Some of these metabolic effects occur pretty quickly," Mehra notes.
The studies reviewed in the current report were all small and of short duration. But larger, long-term epidemiologic studies have come back with similar results. Nevertheless, the relationship between sleep deprivation and weight gain is still not crystal clear. Obesity itself can contribute to sleep loss. Frequent co-occurring conditions, such as sleep apnea (disrupted breathing during sleep), are large contributors to disrupted and poor-quality sleep in the general population. So is obesity causing sleep deprivation, rather than the other way around? "There could be a bidirectional relationship," Mehra says. Although sleep apnea and other conditions can make for low-quality sleep, which can then also lead to heart disease, Mehra suggests that there is ample data that show people who began as normal weight and healthy but did not get enough sleep, over time developed worse health outcomes.