Biological factors are usually the most obvious and well-known factors involved in diseases. There are many different biological factors that can increase the risk of obesity for individuals. Although genetic factors have been linked to obesity, this does not seem to explain the sudden proliferation of obesity that we are seeing today. There is another factor, however, that seems to connect quite perfectly in the scheme of our current generation and obesity: sleep deprivation. Over the past century, the average amount of sleep per night in the United States has plummeted from 9 hours to about 6.5 hours, and seems to be decreasing even more. Recent studies have suggested that maybe the coinciding of the current sleep deprivation epidemic and the obesity epidemic is really no "coincidence" at all. Sleep deprivation has actually been linked to obesity. The research done has not only identified correlation, but specific factors caused by sleep deprivation that can cause obesity. In a study where subjects received 4 hours of sleep, remarkable changes in their hormone levels were noted and their glucose tolerance was impaired (this can be a marker of insulin resistance and diabetes). Sleep deprivation reduced levels of the hormone Leptin by 18%. Leptin is key for the regulation of energy and hunger. Leptin sends signals to the hypothalamus that suppress appetite, so when levels of Leptin are reduced appetite and cravings increase. While Leptin decreased with sleep deprivation, another hunger-regulating hormone, ghrelin, actually increased. Unfortunately, the increase of ghrelin actually has just about the same effect as the decrease of Leptin: an increase in appetite. Ghrelin is a hormone made in the stomach that sends a message to the brain that increases appetite and actually increases cravings for high carbohydrate, high-calorie foods. The craving of these fattening foods caused by ghrelin is likely adaptive and related to evolution and the foods that once kept our ancestors alive and provided them with enough energy, but today these excessive cravings induced by extra ghrelin are certainly not favorable. Not surprisingly, the subjects who were sleep deprived and subsequently had fluctuations of these hormones gained weight. Another study using brain scans further supports the link between sleep deprivation and obesity. A group of neuroscientists and psychologists used functional magnetic resonance imaging (fMRI) to study the brains of two groups of individuals: one who had been sleeping well and one who had not. For those who were deprived of sleep, the fMRI showed increased activity in the more primitive areas of the brain, such as the amygdala. These areas regulate the desire to fulfill the base biological needs: food, sex, and sleep. The more “higher order” areas of the brain controlled in decision making, like the frontal and insular cortexes, showed significantly less activity. So with an increased desire for food and decreased reasoning and decision-making abilities, the outcome is pretty clear. The sleep-deprived subjects wanted junk food. With the ridiculously jam-packed schedules of both adolescents and adults these days, sleep is declining, and clearly at a price. Sleep deprivation and the following hormonal changes clearly illustrate the roles biological factors play in obesity. Hopefully, people can learn from these studies about the importance of sleep and reduce obesity and the various other damaging effects of sleep deprivation.