Lack of Sleep May Lead to Obesity
Some studies have shown that too little sleep is associated with weight gain. This may sound counter-intuitive if you think that we burn more calories during waking activities during the day than at sleep. However, the body has natural cycles and regulation mechanisms that attempts to maintain energy balance and weight. When you throw off this natural cycle by forcing your body to be awake when it wants to be a sleep, or not letting it get the amount of sleep that it desires, then its regulation mechanism may not be as fine-tuned as it should be.
The amount of sleep that one gets affects our hormone levels. In particular, reduced sleep is associated with a greater level of the appetite hormone ghrelin. Insufficient sleep causes our hormones and sugar levels unbalanced, leading to obesity and glucose intolerance.
USA Today reports of a study where a sleep-deprived individuals have greater appetite because they have lower levels of leptin and and higher levels of ghrelin. Leptin is the hormone that gives us the feeling of fullness. Ghrelin is the hormone that increases our appetite.
MSNBC reports similarly saying that people who sleep less than eight hours has more body fat than those who slept more.
Some epidemiological studies show that people who sleep two to four hours a night are 73% more likely to be obese. And those who sleep six hours are only 23% more likely. In comparison, those who get 10 or more hours are 11% less likely to be obese. So the less they sleep, the greater the likelihood of being obese.
Also, if we are tired due to lack of sleep, we have greater difficulty eating healthy; and we grab high calorie foods.
Sleep Deprivation Dysregulation of Appetite
Study found that healthy young volunteers who were deprived of 2 hours of sleep had dysregulation of appetite control. They had higher levels of the hunger hormone ghrelin. And they had less PYY in their blood which suppresses appetite. That causes carbohydrate and sugar cravings.
Study titled Impact of sleep and sleep loss on neuroendocrine and metabolic function had this to say ...
"Laboratory studies in healthy young volunteers have shown that experimental sleep restriction is associated with a dysregulation of the neuroendocrine control of appetite consistent with increased hunger and with alterations in parameters of glucose tolerance suggestive of an increased risk of diabetes"
Dr. Nicholas Perricone writes ...
"sleep deprivation causes us to overeat, because it disrupts the balance between two appetite-related hormones -- ghrelin and leptin." [page 76 of Forever Young]
By sleeping 2 hours less each night, ghrelin the hunger hormone is increased by 15%. Leptin the appetite suppressing and fat store burning hormone is decreased by 15%.
In a podcast, Dan Pardi mentions a study where they sleep deprived individuals and found that ...
"both leptin and ghrelin, which affect feeding, energy regulation, sympathetic nervous system activity — all things that will eventually help to control or influence body weight — would adjust in a manner so that you were more likely to be hungry and more likely to be storing fat, and less likely to be burning energy."
Some estimates are that chronic sleep deprivation may increase risk for obesity by 55%.
Dr. Robert Lustig writes in his book Fat Chance ...
"Sleep deprivation has been shown to increase cortisol and reduce leptin, and in doing so, mimic starvation and hunger. At the brain level, sleep deprivation increases the hunger hormone ghrelin, which increase the "value" each of us puts on food, and also activates the reward system, making you eat even more chocolate cake." [page 69]
Lack of Sleep Lack Impairs Glucose Regulation
Dr. Michael Lucia (in the video on the right) says that lack of sleep causes people to crave carbohydrates the next day -- just like when people are tired will crave carbohydrates. Carbohydrates increase insulin and results in weight gain. Obesity is due not only to dietary effects, but also due to lack of sleep.
Some people without enough sleep will tend to crave nicotine and caffeine the next day. So they end up consuming sugary drinks with caffeine. And sugar increases risk of obesity.
Fat Release Mechanism
Another theory is that our fat release mechanism is related to our sleep cycle. The cells in your body needs a constant supply of fuel even during sleep. Some of this fuel comes in the form of fatty acids from metabolized foods. However since we cannot be eating during sleep, the body puts the fuel into temporary storage in fat cells so that this fuel can be released for use during the night. During that day when we eat, some of these fatty acids goes into fat cells where it is bound up into triglycerides. A triglyceride is three fatty acids bonded together by a glycerol molecule. During the night, the body releases the bonds of this triglyceride and enabling the fatty acids to flow out of the fat cell into the bloodstream where it is distributed to the cells that needs it. Well, if your night is artificially shorten by not sleeping enough, then not as much of the triglyceride in the fat cell will get released.
This idea comes from the study of rats. The book Good Calories, Bad Calories says ...
"While rats are sleeping, they progressively mobilize more and more fatty acids from their adipose tissue and use these fatty acids for fuel." [page 431]
Note that what is described may be a very small effect in the big scheme of things. Weight gain is caused by many other factors besides sleep. And in particular, sugar and high glycemic carbohydrates (such as flour and bread) are much bigger contributors to weight gain. Insulin resistant conditions can also affect weight gain.
 Blood Sugar Solution by Dr. Mark Hyman
 Why Most People are Sleep Deprived - Chris Kresser podcast.