Light and your biological clock

The biological clock can be shifted bu light. Bright light exposure of our eyes in the morning shifts the clock forward. That means that the clock tells the body tomorrow at an earlier hour that it is time to wake up. Bright light exposure in the evening has the opposite effect: the biological clock indicates the following day at a later moment that it is time to get up. This knowledge can be used to influence the biological clock with light therapy. This can be desirable to reduce or prevent jetlag complaints and to bring forward very late sleeping times or to shift very early sleeping times to a later moment.

Morning- and evening persons
People differ in their preference for bed times. Everybody knows the difference between morning risers and evening persons. In the case that someone is an extreme evening person difficulties may arise, like extreme tiredness, not being able to keep start times for work, or even depressed mood and social disfunctioning. This sleep disorder is called Delayed Sleep Phase Syndrome (DSPS). In the opposite case, someone how goes to bed and get up extremely early, the sleep disorder is called Advanced Sleep Phase Syndrome (ASPS).
 
Extreme morning people can shift their sleep to a later moment by light exposure in the evening. Then they will stay up longer in the evening and wake up less early, at more preferable times. Extreme evening people can shift their sleep to an earlier momen by extra light exposure in the morning. Then they will have less problems getting up at more accepted times. To shift the biorhythm natural daylight or artificial lighting can be used.

PDF: Incandescent replacement lamps and health, v. Bommel, 2010