The psychophysiology of shift work is mainly related to circadian rhythmicity and sleep-wake phenomena. Individuals on a rotating three-shift or similar system work the night shift at the low phase of circadian rhythm. On retiring to bed in the morning they fall asleep rapidly but are prematurely awakened by their circadian rhythm and exhibit severe sleepiness and reduced performance capacity. In connection with the morning shift the circadian psychophysiology makes it difficult to fall asleep as early as needed during the preceding night. Around 0400 to 0500, when the individuals should rise, they have difficulties awakening because of the sleep loss and the circadian rhythm, which at that point is at its lowest. Subsequently, day work is characterized by sleepiness and reduced performance. It should be emphasized that it does not seem possible to improve one's ability to adjust over time, even with permanent night work. Older age and "morningness" personality are related to higher than average problems in adjusting.