The literature on sleep and obesity is becoming dense. Lots of things happen to people when they sleep. One of them has to do with appetite regulation, so many researchers are coming to believe that sleep plays a dominant role in today’s vast American Overfeed.
This hunch is supported by striking correlations.
In 1960, a survey of over 1 million people found a modal sleep duration of 8-9 hours. In 2002, polls conducted by the National Sleep Foundation indicated that the average duration of sleep for Americans had fallen to 6.9-7 hours. Recent data indicate that a higher percentage of adult Americans report sleeping 6 hours or less. In 2005, in the US, more than 30% of adult men and women between the ages of 30 and 64 years reported sleeping on average less than 6 hours each night. This decrease in sleep duration has occurred over the same time as the increase in the prevalence of obesity and diabetes.
Leptin has a distinct diurnal and circadian rhythm. It has minimum values during daytime and a nocturnal rise with maximum values during early to mid sleep. The amplitude of the circadian variation averages approximately thirty per cent. Leptin levels rise during the night to suppress appetite while sleeping. Moreover, the reduction of leptin at night spells bad news for the rest of the day: it sets the individual up not just with lower leptin levels in general but also decreased glucose tolerance and an increased craving for carbohydrates.
In order to test the effects of sleep deprivation on leptin production, a number of studies have been conducted. They’re all fascinating, so I have provided a quick review of some of the more revealing studies.
1) Many studies are conducted on people with sleep apnea. Epidemiological studies show that they are heavier than the rest of the population. They have greater rates of diabetes and metabolic syndrome. Yet when their sleep apnea is corrected, these people lose weight naturally, and their metabolisms normalize. This probably has to do both with decreased appetite as well as improved metabolic functioning.
2) In one study at the University of Chicago, doctors measured levels of leptin and ghrelin in 12 healthy men. They also noted their hunger and appetite levels. Soon after, the men were subjected to two days of sleep deprivation followed by two days of extended sleep. During this time doctors continued to monitor hormone levels, appetite, and activity. The end result: When sleep was restricted, leptin levels went down and ghrelin levels went up. Not surprisingly, the men’s appetite also increased proportionally. Their desire for high carbohydrate, calorie-dense foods increased by a whopping 45%.
3) In another study at the University of Chicago, a similar protocol was conducted but men were asked to return a year later for a comparison. For six days they got four hours of sleep — their week of sleep deprivation. The men’s food and activity levels were strictly regulated and hormone levels were taken during the day and while they slept. One year later, the men returned for a six-day study with an 8-hour sleep period, so they served as their own comparison group. The results: After their six-day sleep deprivation period, volunteers had a leptin decrease ranging from 19-26 percent.
4) In another study — a joint project between Stanford and the University of Wisconsin — about 1,000 volunteers reported the number of hours they slept each night. Doctors then measured their levels of ghrelin and leptin, as well as adiponectin, insulin, glucose, a lipid profile, and they also charted their weight. The result: Those who slept less than eight hours a night not only had lower levels of leptin and higher levels of ghrelin, but they also had a higher level of body fat. What’s more, that level of body fat seemed to correlate with their sleep patterns. Specifically, those who slept the fewest hours per night weighed the most.
5) In the final study, young, healthy subjects who were studied after 6 days of sleep restriction where they were allowed four hours in bed. After full sleep recovery, their levels of blood glucose after breakfast were higher in the state of sleep debt despite normal or even slightly elevated insulin responses. The difference in peak glucose levels in response to breakfast averaged was large enough to suggest a clinically significant impairment of glucose tolerance. These findings were confirmed by the results of intravenous glucose tolerance testing. Indeed, the rate of disappearance of glucose post injection was nearly 40 per cent slower in the sleep-debt condition than after recovery, and the acute insulin response to glucose was reduced by 30 per cent.
How fast do leptin levels recover from sleep deprivation?
Leptin levels recover almost as soon as regular sleep is resumed, at least in controlled studies. In the first night. However, these studies occur over a week or two at most. If the sleep deprivation is chronic, it seems to have the same effect as fasting does on leptin. Levels remain low–at least for some time–despite resumption of “normal” sleep or eating. It takes time for the system to re-equilibrate after chronic stressors.
How does stress act on this system?
Stress activates cortisol secretion, but it also stimulates sympathetic nervous system activity. This gets adrenaline running in the system, increases heart rate, and increases blood pressure. These two things increase during both partial and acute sleep deprivation. It is well kown that andrenergic (adrenal-related) receptor activation is suppressive of leptin production, and that leptin is reduced in response to adrenaline infusions. For this reason, whatever dampening that stress puts on sleep negatively affects appetite activity.
There are other downstream effects of sleep deprivation. I’ll cover some of them briefly here, then hopefully return to them each on their own.
1) More cortisol dysregulation. One effect of sleep deprivation is a decrease early evening cortisol levels. Normally at that time of day, cortisol concentrations are rapidly decreasing in order to attain minimal levels shortly before habitual bedtime. Yet in one study the rate of decrease of cortisol concentrations in the early evening was approximately 6-fold slower in subjects who had undergone 6 days of sleep restriction than in subjects who were fully rested. This means, basically, that it takes longer for people who have lost sleep to ramp down from that stress and be able to go to sleep again.
2) Thyroid reduction. In one study, after 6 days of 4-hour sleep time, people experienced a striking decrease in the normal nocturnal TSH rise, and the overall mean TSH levels were reduced by more than 30%. A normal pattern of TSH release reappeared when the subjects had fully recovered. T4 was higher in the sleep-restricted condition than the normal condition, indicating that decreased sleep decreases the body’s rate of conversion from T4 to T3.
3) Growth hormone reduction. The temporal organization of Growth Hormone secretion is also altered by chronic partial sleep loss. The normal single GH pulse occurring shortly after people fall asleep splits into 2 smaller pulses, 1 before sleep and 1 after sleep. With decreased sleep, peripheral tissues are exposed to high GH levels for an extended period of time. GH has anti-insulin-like effects, so an increased overnight exposure to GH negatively impacts insulin sensitivity and glucose tolerance.