The Link Between Sleep & Obesity
A growing body of research has uncovered a strong link between inadequate sleep duration and obesity in both children and adults. Although conventional wisdom might lead one to believe that less sleep time would result in increased energy expenditure, studies have shown that sleep deprivation can influence appetite and food choices, leading to weight gain.
A recent study by St-Onge et al, examined the neuronal response to unhealthy foods and sleep restriction. For five consecutive nights healthy, normal weight subjects spent either four hours or nine hours in bed. After the five night period, a magnetic resonance imaging (MRI) test was performed while subjects were in a fasted state. During the MRI, subjects were presented with healthy and unhealthy food stimuli. When subjects spent five nights with only four hours in bed, there was much higher neuronal activation in response to unhealthy foods. This increase of activity occurred in parts of the brain that are linked to reward and food-sensitive centers. 
Another study examined subjects that had either experienced a normal amount of sleep for eight consecutive nights or 2/3 of their normal sleep time for eight consecutive nights. Researchers measured caloric intake, energy expenditure, and blood levels of leptin and ghrelin, hormones that are responsible in regulating appetite. Subjects on the restricted sleep schedule had a higher caloric intake by over 600 kcal per day. There were no differences in caloric expenditure or leptin and ghrelin levels between the groups. This increase in caloric intake without a change in caloric expenditure could contribute to weight gain or obesity in people that experience sleep restriction for an extended period of time. 
While regular physical activity and a healthy diet still remain the keys to maintaining a healthy body weight, these studies demonstrate that adequate sleep time also plays an important role in obesity and obesity-related chronic disease prevention.