Light is one of the most important environmental factors that can affect sleep. It does so both directly, by making it difficult for people to fall asleep, and indirectly, by influencing the timing of our biological clock and thereby affecting our preferred time to sleep.
The direct effect of light is to suppress the levels of the hormone melatonin, which plays a role in initiation of sleep. Even fairly dim light, such as the brightness of a television or computer screen, can be enough to interfere with sleep.
Light also influences our biological clock through specialized “light sensitive” cells in the retina of our eyes. The purpose of these cells is to tell the brain whether it is daytime or nighttime, and the timing of our sleep patterns are set accordingly. Artificial light close to bedtime on a regular basis may delay the time we are able to fall asleep and also to wake up in the morning.