A 2005 study done at Columbia University showed that the less sleep the subjects got, the more likely they were to be obese. Those who got only four hours or less of sleep a night were 73% more likely to be obese than those getting seven-nine hours per night. Similarly, those sleeping five hours per night were 50% more likely to be obese and six hours per night were 23% more likely to be obese than those getting seven to nine hours of sleep per night. The authors pointed out that the lack of sleep actually changed the hormone levels, creating increased hunger and cravings for sweets and carbohydrates!
The sleep-curtailment condition resulted in decreased leptin levels (signaling that you are satisfied), increased ghrelin levels (signaling that you are hungry), and markedly elevated hunger and appetite ratings. The subjects were found to particularly crave sweets, starch, and salty snacks after being deprived of sleep.
To read more, see the actual study attached. Sleep and Obesity