Most biologic and psychological processes vary according to a natural rhythm. Many of these functions have a cycle of about one day and are called circadian rhythms.

Circadian rhythms influence body temperature, sleep and wakefulness, and a variety of hormonal changes. Sunlight and other time cues help to set circadian cycles so that they are consistent from day to day. Even if we didn't have time cues from our environment, fluctuations in circadian rhythms would continue to occur within a period of a little more than one day (generally about 25 hours). As a consequence of this we "reset" our internal clocks on a daily basis by going to sleep at the same time each night and awakening at the same time each day.

Circadian rhythms are coordinated by a group of cells at the base of the brain called the suprachiasmatic nucleus (SCN). The SCN has connections with other parts of the brain that control the body's temperature, release of hormones, and many other automatic functions. A pathway runs from the eye directly to the SCN which explains why light seems to be the most important cue in terms of "setting" our internal clock. Interestingly, blind people often report problems with circadian rhythms, since they cannot get the visual time cues that sighted people can get by just turning on a light. Other factors that may affect the SCN and the setting of our circadian clock include changes in our body temperature caused by environmental changes or by the presence of fever that accompanies different illnesses and by the effects of certain medications.

In healthy people, the various circadian rhythms are "in tune" like the many instruments of an orchestra. Body temperature, for example starts to rise during the last hours of sleep, just before waking up in the early morning. At night, body temperature starts to fall which aids in sleep onset. A drop in temperature also occurs in most people between 2:00 and 4:00 p.m., which may explain why people feel sleepy in the early afternoon and why many people take an afternoon "siesta."

• Follow a regular schedule of going to bed and waking up in the morning all week long. Do not "sleep in" on weekends or holidays.

• Avoid daytime naps, even if you miss sleep the previous night.

• Avoid drinking caffeine containing beverages such as coffee, tea or colas after 7:00 p.m. Chocolate also contains caffeine.

• Make sure your sleeping environment is comfortable. This includes having a quiet room, a comfortable bedroom temperature and a comfortable bed.

• Use your bed only for sleep or intimacy with your significant other.

• Do not eat meals in bed or conduct business from bed.

• Do not watch TV or read in bed if you experience any problems falling or staying asleep.

• Associate your bed with sleeping, relaxation and comfort.

• Engage in moderate exercise in the early part of the day; try to avoid strenuous physical activity within three hours of going to sleep.

• Develop a positive attitude about sleep.

• Get out of bed and engage in a relaxing activity if you are unable to fall asleep within 15 to 20 minutes of turning out the lights.

• Do not use your time in bed to worry about problems. Sleep is not something that you can accomplish by "hard work." It will occur naturally if you allow it to.