People who don't get enough sunlight are likely to suffer from vitamin D deficiency. Those living in areas of few sunlit hours are more subject to depression as happens in Alaska where winter days offer only a few hours of sunlight, if any at all. Those who work the night shift must make what is often a difficult adjustment to sleeping during the day rather than at night. Too much light also carries problems. It disrupts growth cycles in plants, and can cause health problems in people.
Light influences health because it affects the physiologic or physical functions of the body.
Light cycles can change body temperature, for example, which will cause changes in hormone production. The body naturally adapts to cycles of light and dark, or day and night. This automatic ability is called Circadian rhythm, or the body's inner clock. The body clock is affected when individuals can travel around the globe through eight time zones in a single day, turning what was their period of daylight into the darkness of night. People who have had that experience know the discomfort of trying to readjust their life cycles to a new set of light and dark hours.