The cycle is anchored in part by ambient lighting (darkness causes a person's body to release the hormone melatonin, which induces sleep),[7] and by a person's imposed pattern of regular sleeping and waking times. The influence of the day-night cycle is never fully displaced (artificial lighting inhibits melatonin release more weakly than sunlight),[8] 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.