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During the past century sleep durations have fallen while obesity rates have risen. Coincidence?
Sleep restriction results in lower glucose levels even during subsequent recovery sleep. The insulin levels are elevated after sleep restriction. Insulin-to-glucose ratio increases, although after enough recovery sleep it eventually returns to normal. This variation in blood sugar argues against a pattern of varying sleep time. You can’t totally justify a night or two of short sleep by saying you’ll make up for it with extra sleep the next day or two. This might bring your brain’s sleep needs back into equilibrium, but during this time your body has had too much sugar floating around. Over a prolonged period this type of behavior could raise the chances of getting Type II diabetes. People who burn the candle at both ends during the work week and think they can recover on the weekends should take note.
Large meals make people sleepy. There are evolutionary explanations for this: when our ancestors had acquired enough food they had time to sleep, and sleep is a time when the body releases higher levels of growth hormone – kids actually grow during sleep – so the body needs nutrients. Hunger, if bad enough, prevents sleep. Our ancestors were not supposed to sleep until they put some food in their bellies.
It is also true that sleep stops – or at least reduces – feelings of hunger. That is why we can go for a much longer period of time without eating overnight (between dinner and breakfast the next morning) The hormone orexin, or hypocretin plays a key role in keeping us awake, and it also appears to be tied to the appetite cycle. The name orexin actually derives from the Latin word for “to eat” and hypocretin is derived from “hypo” (from hypothalamus, a part of the brain) and “cretin” (a gut hormone). This hormone was discovered separately by two research teams (hence the two names for the same hormone). Both teams were primarily interested in the physiology of eating and found the hormone when looking into appetite.
Leptin and ghrelin both affect elements of the complex immune system. Nobody really knows how it all fits together, but this suggests a mechanism for short sleep and its effect on exacerbating autoimmune and inflammatory diseases.
A potential explanation for the relationship between sleep and weight relates seasonal changes in daylight time to availability of food. Our ancestors ate more in the summer when food was plentiful. They also slept less with the short nights. During the winter, they both cut food consumption due to fewer edible resources in the environment and slept more to conserve energy.
Obesity can contribute to apnea and make it difficult to get a comfortable position on the bed. Pickwickian syndrome is a recognized form of sleep apnea disorder suffered by moderately to severe obesity. But even for slightly overweight people there is an interaction between sleep debt and weight.
Chronic sleep debt may promote metabolic syndrome. It increases stress markers in the body, makes you eat more, and tends to mess up your blood sugar levels. Indeed, sleep onset insomnia is now considered a risk factor for hyperglycemia.
People with apnea have worse sleep and more nighttime ghrelin in their blood than those without apnea at the same BMI. Sleep deprivation or restriction not only increases appetite, but it decreases glucose tolerance. Indeed, epidemiological studies have shown chronic short sleep and chronic sleep are associated with type II diabetes. A single night of sleep restriction raises levels of cortisol (the “stress hormone”) and leptin the next morning.
A large-scale study at Case Western Reserve University (more than 68,000 women) found that those who sleep less than 5 hours a night gain more weight over time than those who sleep 7 hours a night. The women who slept less were more likely to become obese, and here’s something that may be counterintuitive: the women who slept less consumed fewer calories than the ones who slept a full night, on average.
This finding goes against the common wisdom that overeating among the sleep-deprived explains such weight differences. Actually, there are other studies that show similar results, but a study reported in the journal Pediatrics in 2007 casts some doubt on the connection or correlation. This study looked at kids aged 10 to 19 and found that when very detailed diaries of sleep time were kept, there wasn’t as much of a relationship between weight and sleep time. This does not mean that studies linking sleep loss to excess weight are wrong, but it does point out the difficulty of measuring things as elusive as time spent sleeping every night.
If insomnia is caused by too much arousal (as it often seems to be), the body is revved up too much. An insomniac has a higher metabolic rate than a good sleeper. Measurements of VO2 have found “whole body VO2, measured at intervals across the day and throughout one night of sleep, was consistently elevated at all measurement points in the insomniacs as compared to the normal”.
Adolescents also gain weight if they don’t sleep enough. Given the fact that teens need more sleep than adults and the increasing obesity in the whole population, including those under 18, this is worrisome from a public health standpoint.
The notoriously bad diets of teens may be exacerbated by insufficient sleep and sleep restriction leads kids to eat more carbohydrates People with noctural eating syndrome also favor carbohydrates. At the other end of the lifespan, it is estimated that 77% of older Americans who are obese have sleep problems.
Insomnia and insufficient sleep have a negative effect on glucose tolerance, so much so that the some scientists now regard sleep onset insomnia as a risk factor for hyperglycemia,
Scientists did detailed analysis of nocturnal levels of ghrelin and leptin in the bloodstream. Comparing the levels for insomniacs with those for normal people, they found ghrelin levels were about 30% lower in those with primary insomnia. Leptin levels were the same in both groups.
Ghrelin makes you hungry, so decreased ghrelin levels should lead to decreased appetite and weight loss, right? No. This is one of the seemingly paradoxical effects the body’s homeostatic systems often produce. During the day the sleep loss causes increased levels of ghrelin and decreased levels of leptin. This increases the appetite. The sleep deprivation caused by insomnia causes a dysregulation in the body’s “energy balance” which causes the body to take in more energy than it needs. In other words, it makes you gain weight.
Circadian cycles are so important in sleep regulation and behavior. There is also evidence that the time of day when eating occurs has an impact on weight. Generally, the later during the day you eat, the fatter you tend to be.
Studies have shown that both short-term (one or two nights) and long-term insufficient sleep promotes weight gain. The body does not adjust fully to the short sleep. The increased appetite may be evident more in acute deprivation, but over the long haul the body’s weight will stabilize at a higher level than if the person slept a healthy amount.
It should be noted that this correlation between short sleep (habitual or otherwise) and weight gain is found in people who are sleeping less than they normally would with no external pressures (e.g. job, family, etc.) Some people seem to have a genetic tendency to insomnia in all environments and those people tend on average to be thin.