Sleep deprivation is a key risk factor for weight growth and obesity. Poor sleep has been frequently related to an increase in body mass index (BMI) and weight gain (2, 3, 4). People's sleep needs differ, but studies have shown that getting less than seven hours of sleep every night causes weight gain (3). Long term sleep loss may also be associated with increased appetite and decreased physical activity, which can further contribute to weight gain (3). Studies have shown that sleep-deprived people tend to eat more than they would if they were not sleep deprived (5). This could explain, at least in part, why those who work nights or who are required to stay awake for long periods of time tend to be more likely to suffer from obesity. Sleeping too little can also lead to stress eating; this is because when you're tired your body uses up its energy rather than storing it as fat because it thinks you need it for survival. Stress also causes us to eat foods that aren't good for us - such as sweets and fatty foods - so if you are not getting enough sleep, then this will put you at risk of becoming overweight or obese. There is some evidence to suggest that being sleepy all the time can also lead to weight gain. One study showed that women who reported sleeping only four hours or less per night on average weighed 11 pounds more than women who said they slept more than this amount (6).