A study from the University of California Berkeley found that people who are sleep-deprived have a harder time resisting unhealthy foods than people who have had a good night’s sleep.   

Researchers tracked the blood flow to the brain of 25 men and women of normal weight while they were shown pictures of healthy and unhealthy foods.  The subjects were scanned once after getting nine hours of sleep each night for five nights, and once after getting four hours of sleep each night for five nights.

Brain scans showed that when people were in a state of sleep-deprivation, the images of unhealthy foods activated the reward center of the brain, making the foods harder to resist.   

The same pictures of unhealthy foods did not activate the reward centers of people who had adequate sleep.