" You probably know firsthand that sleep affects mood. After a sleepless night, you may be more irritable, short-tempered, and vulnerable to stress. Once you sleep well, your mood often returns to normal. Studies have shown that even partial sleep deprivation has a significant effect on mood. University of Pennsylvania researchers found that subjects who were limited to only 4.5 hours of sleep a night for one week reported feeling more stressed, angry, sad, and ...