Topic Overview

What is the body clock?

The body's "biological clock," or 24-hour cycle (circadian rhythm), can be affected by light or darkness, which can make the body think it is time to sleep or wake up. The 24-hour body clock controls functions such as:

• Sleeping and waking.
• Body temperature.
• The balance of body fluids.
• Other body functions, such as when you feel hungry.

How are body clock problems and sleep problems connected?

Body clock sleep problems have been linked to a hormone called melatonin. Light and dark affect how the body makes melatonin. Most melatonin is made at night. During the day, light tells your body to make less melatonin. If you work at night in artificial light, your body may be making less melatonin than it needs.

Some people—such as those who can't sleep until very late and those who go to bed very early—have circadian (say "ser-KAY-dee-un") rhythms that are different than those of most people. Other people with sleep problems may have regular circadian rhythms but have to adjust them to new situations, such as working a night shift.

What sleep problems are related to problems with your body clock?

Things that may affect melatonin production and can cause sleep problems include:

• Jet lag. Crossing time zones disrupts your body clock. You have sleep problems because your body clock has not adjusted to the new time zone. Your body thinks that you're still in your old time zone. For example, if you fly from Chicago to Rome, you cross seven time zones. This means that Rome is 7 hours ahead of Chicago. When you land in Rome at 6:00 in the morning, your body thinks it's still in Chicago at 11:00 the previous night. Your body wants to sleep, but in Rome the day is just starting.
• Changing your sleep schedule. When you work at night and sleep during the day, your body's internal clock needs to reset to let you sleep during the day. Sometimes that's hard to do. People who work the night shift or rotate shifts may have trouble sleeping during the day and may feel tired at night when they need to be alert for work.
• Your sleep environment. Too much light or noise can make your body feel like it is not time to sleep.
• Illness. Certain illnesses and health problems can affect sleep patterns. These include dementia, a head injury, recovering from a coma, and severe depression. Some medicines that affect the central nervous system may also affect sleep patterns.
• Aftereffects of drugs and alcohol. Some drugs cause sleep problems. Also you may fall asleep with no problems after drinking alcohol late in the evening, but drinking alcohol before bed can wake you up later in the night.