Circadian rhythm
Circadian rhythm effects describe the tendency for humans to experience a normal cycle in attentiveness and sleepiness through the 24-hour day. Those with a conventional sleep pattern (sleeping for seven or eight hours at night) experience periods of maximum fatigue in the early hours of the morning and a lesser period in the early afternoon. During the low points of this cycle, one experiences reduced attentiveness. During the high points, it is difficult to sleep soundly. The cycle is anchored in part by ambient lighting (darkness causes a person's body to release the hormone melatonin, which induces sleep), and by a person's imposed pattern of regular sleeping and waking times. The influence of the day-night cycle is never fully displaced (standard artificial lighting is not strong enough to inhibit the release of melatonin), and the performance of night shift workers usually suffers. Circadian rhythms are persistent, and can only be shifted by one to two hours forward or backward per day. Changing the starting time of a work shift by more than these amounts will reduce attentiveness, which is common after the first night shift following a "weekend" break during which conventional sleep times were followed.