Sleep is also a time when metabolic functions change and slow down. Some of these metabolic changes help regulate appetite. Appetite is controlled by the hormones gherlin and leptin. Leptin is stored in and released from fat tissue, and it inhibits appetite and increases energy. Ghrelin, on the other hand, is a hormone made in the stomach which increases appetite and lowers energy levels. Studies have shown that sleep deprivation causes almost 20 percent decline in leptin and increase in gherlin levels as much as 28 percent. This means that lack of sleep correlated with hungrier. Another study showed that less sleep correlated with higher BMI, possible due to the fact that hungrier people eat more.
Another important effect of not sleeping is a decrease in immunity. Studies show that people who are deprived of sleep from 3 am-7 am experience a 30% drop in immunity and a 50% drop in immune function for up to 4 weeks. So it is very important to get regular sleep every day.