Suppression of melatonin secretion in some blind patients by exposure to bright light

The eyes of some totally blind persons may convey enough information about light and darkness to their brains to synchronize their internal circadian rhythms. A study of 11 blind patients who did not consciously perceive light and of six normal subjects found that three of the blind patients experienced decreases in plasma melatonin concentrations during exposure to bright light, as do sighted persons. The other eight blind patients did not experience changes in their plasma melatonin concentrations. Plasma concentrations of melatonin are found to be higher during nighttime than during the day, and are a way of synchronizing, or entraining, the circadian rhythm to the 24-hour day. Blind persons who do not perceive light may experience periods of severe insomnia because they do not receive the cues of light and dark to entrain their bodies to the 24-hour day.