Sleep Duration Affects Appetite-Regulating Hormones

Published: December 7, 2004

Some of us, when awake in the middle of the night, feel an urge to visit the kitchen. This could explain results of previous studies that have shown a link between short sleep duration and high body mass index (BMI). But a study by Emmanuel Mignot and colleagues suggests that it's not just the additional snacking opportunities that make short sleepers more likely to be overweight.

Intrigued by the connection between sleep and BMI, and by recent studies showing that sleep deprivation in laboratory settings can cause a decrease in serum levels of leptin, a hormone known to control appetite, Emmanuel Mignot and colleagues set out to study the levels of various hormones known to regulate appetite and energy expenditure under “real life” conditions.

They took advantage of the Wisconsin Sleep Cohort Study, an ongoing longitudinal study of sleep habits and disorders in the general population. The study began in 1989, when researchers mailed state employees aged 30–60 years a survey on sleep habits, health, and demographics. Mail surveys were repeated at 5-year intervals, and some of the respondents were recruited to sleep a night in the laboratory and undergo various tests. A number of participants were also asked to keep a sleep diary for 6 days. The study has already shown connections between sleep apnea and hypertension, and between menopause and sleep-disordered breathing.

For their study, Mignot and colleagues measured sleep duration (habitual and immediately prior to blood sampling), BMI, and pre-breakfast blood hormone levels in 1,024 participants. Consistent with previous studies, they found that in individuals who sleep less than 8 hours (74% of all participants), BMI was inversely proportional to sleep duration. In addition, short sleep was associated with low leptin and high ghrelin levels (ghrelin is a hormone thought to stimulate food intake).These hormonal differences are likely to increase appetite, which could be responsible for the increased BMI in short sleepers.

These findings could explain, at least in part, why societies in which excess calories are much easier to come by than a good night's sleep are more prone to obesity. Mignot and colleagues plan to test this in intervention studies where they make people sleep more and measure the effects on body mass. “Good sleep, healthy eating habits, and regular exercise each may have important roles in fighting obesity in modern society,” suggests Mignot.

The Body's Thermoregulation During Sleep

Human beings are endotherms - able to thermoregulate - that is, maintain their body temperature. Body temperature is regulated through a balance of heat absorption, production and loss. Human temperature must be maintained within a fairly small range, up or down from the resting temperature of 98.6. Temperatures above 104.9 degrees Fahrenheit or below 92.3 degrees generally cause injury or death.

Humans have two zones to regulate, their core temperature and their shell temperature. The temperature of the abdominal, thoracic, and cranial cavities, which contain the vital organs, is called the core temperature. Core temperature is regulated by the brain. The shell temperature includes the temperature of the skin, subcutaneous tissues, and muscles, and it is more affected by external temperature. The core is able to conserve or release heat through the shell.

When the core temperature is too high, blood vessels in the skin dilate and heat is lost through their walls. Sweat is also produced, which evaporates and lowers temperature. If a human is too cold, the blood vessels constrict, conserving heat. Blood is preferentially shunted to the internal organs and away from the skin and peripheral structures like limbs.

The hypothalamus regulates body temperature between 96.8 and 100.4 degrees Fahrenheit over each 24 hour cycle. During the normal human circadian rhythm, sleep occurs when the core temperature is dropping. Sleep usually begins when the rate of temperature change and body heat loss is maximal. The average adult’s lowest temperature is at about 5 AM, or two hours before waking time.

Many mammals lose significant thermal regulatory capacity during sleep. Some animals like squirrels go into a torpor state during sleep, in which their body temperature dips well below the normal level for hours at a time. However, most research to date seems to indicate that humans do not have significant difficulty thermoregulating during sleep.

In one study, subjects were exposed to a range of temperatures during sleep. Based on animal models, the researchers expected REM sleep to cause difficulty in thermoregulation, but the results showed that there was very little disruption of thermoregulation during REM and other sleep stages. The subjects shivered slightly in cold temperatures during sleep stages 1 and 2. Although skin temperature increased as the subjects were exposed to higher temperatures, their core temperature readings did not change.

A recent Dutch study shows just how important temperature is when it comes to sleep quality and fragmentation. Fitting human subjects with thermosuits, the scientists were able to lower skin temperature less than a degree Centigrade without affecting core body temperature. The changes were dramatic. People didn't wake up as much during the night and the percentage of the sleep spent in stages 3 and 4 (deep sleep) increased. The effects were most pronounced in the elderly and in people who suffered from insomnia. A 0.4 C decrease in skin temperature caused a decline in the probability of early morning waking from 0.58 to 0.04.

The same researchers found that people with narcolepsy tend to have higher skin temperature when asleep, and also when awake. They speculated that that hypocretin deficiency in narcolepsy affects skin-temperature regulation.

Other studies have showed different thermoregulatory responses of human subjects, depending on the sleep stage and temperature of the environment. In a different study of adult humans, thermoregulatory efficiency during REM sleep was fairly well maintained. However, thermoregulation was less efficient during Slow Wave Sleep (SWS). When subjected to different environmental temperatures, regulatory processes were affected. An overly warm or cool temperature disturbed sleep. REM sleep decreased, as did SWS to a lesser extent.

However, warmth beforehand improved sleep, especially SWS. In depressed patients, sleep is disturbed as well as body temperature rhythms. In these patients, a warm temperature before sleep might be helpful.

It does seem that humans maintain thermoregulation during sleep. However, it is possible that ambient temperatures before sleep may have an effect on sleep initiation and quality.

Here's an interesting fact: you don't sweat or shiver during REM sleep. Sleep researcher Jim Horne compares the REM non-thermal regulation period to that of normal functioning of babies, who neither sweat nor shiver even when awake. Babies control their body temperature, when it gets too cold, not by shivering but by use of so-called "brown fat" which is a type of adipose tissue well suited to generating heat. Adults have substantially less brown fat, adjusting for body weight, than babies do, but Horne thinks it is possible that adults use brown fat to keep from cooling too much during REM.

Manipulating your body temperature to get to sleep

You really can't change your body temperature much without getting severely ill. It is very dangerous if you temperature goes more than a few degrees above or below normal. However, many find that cooling down helps them get to sleep. Why does a warm (but not hot) bath help so many get to sleep? Because it ends up cooling you down, especially as you dry off and the residual water on your skin evaporates. Recent research by Dutch scientists found that by increasing skin temperature the sleep quality in elderly people could be enhanced. People in the test wore heated thermosuits and with a slight (half a degree) increase in skin temperature were ale to increase the length of time spent in slow wave sleep and decrease incidences of waking.

Impaired breathing during sleep can disrupt memory and thinking

Researchers say new data from brain image scans shows that patients who suffer from sleep apnea experience loss of brain tissue in the memory storage areas of the brain.

When someone suffers from sleep apnea, their breathing is periodically stopped by muscle and other types of tissue. These pauses in breathing can last anywhere from a few seconds to a few minutes. Many times, they happen between five and 30 times each hour, and in some severe cases even more often. After the breathing pause is over, normal breathing resumes, occasionally with loud snorting or choking sounds.

For most sleep apnea sufferers, it is a chronic ailment that interrupts your sleep three or more nights every week. When one is suffering from sleep apnea, he or she typically moves from deep sleep to light sleep whenever a breathing pause occurs.

"Our findings demonstrate that impaired breathing during sleep can lead to a serious brain injury that disrupts memory and thinking," principal investigator Ronald Harper of the David Geffen School of Medicine at the University of California, Los Angeles said.

Harper and his team used a technique called magnetic resonance imaging to gather high resolution pictures the 43 patients’ brains, including cross sections of mammillary bodies. Mammillary bodies are clumps of tissue on the underside of the brain, and are so named for their resemblance to breasts. The researchers compared the results to pictures from 666 control subjects matched for gender and age.

Harper and his team, who published their findings in the journal Neuroscience Letters, found that in patients who suffered from sleep apnea, the brain’s mammillary bodies were nearly 20 percent smaller than the control group’s.

"The findings are important because patients suffering memory loss from other syndromes, such as alcoholism or Alzheimer disease, also show shrunken mammillary bodies," study lead author Rajesh Kumar said.