structural:scholar_edition
Lack of sleep

Sleep has a complex, and as yet not fully elucidated, relationship with mood. Most commonly if a person is sleep deprived he/she will become more irritable, angry, more prone to stress, and less energized throughout the day. "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 mentally exhausted. When the subjects resumed normal sleep, they reported a dramatic improvement in mood." Generally, evening oriented people, as compared to morning ones, show decreased energy and pleasantness and heightened tension.

However, in a subset of cases sleep deprivation can, paradoxically, lead to increased energy and alertness and enhanced mood. This effect is most marked in persons with an eveningness type (so called night-owls) and people suffering from depression. For this reason it has sometimes been used as a treatment for major depressive disorder.