Light dependence
Production of melatonin by the pineal gland is inhibited by light and permitted by darkness. For this reason melatonin has been called "the hormone of darkness" and its onset each evening is called the Dim-Light Melatonin Onset (DLMO). Secretion of melatonin, and its level in the blood, peaks in the middle of the night, and gradually falls during the second half of the night, with normal variations in timing according to an individual's chronotype.
Until recent history, humans in temperate climates were exposed to up to 18 hours of darkness in the winter. In the modern world, artificial lighting reduces this to typically eight hours or less per day all year round. Even low light levels inhibit melatonin production to some extent, but over-illumination can create significant reduction in melatonin production. Since it is principally blue light that suppresses melatonin,[19] wearing glasses that block blue light [20] in the hours before bedtime may avoid melatonin loss. Use of blue-blocking goggles the last hours before bedtime has also been advised for people who need to adjust to an earlier bedtime, as melatonin promotes sleepiness.
Reduced melatonin production has been proposed as a likely factor in the significantly higher cancer rates in night workers,[21] and the effect of modern lighting practice on endogenous melatonin has been proposed as a contributory factor to the larger overall incidence of some cancers in the developed world.[22] As inadequate as blood concentrations may be in brightly lit environments, some scientists now believe that a person's overnight output of melatonin can be further jeopardized each time he or she interrupts his or her sleep and turns on a bright light (suggesting that using a less-bright nightlight would be safer). Others suggest that such short exposures do no harm.[23]