Circadian rhythms are 24-hour cycles in the physiological, behavioral or biochemical processes of living things: plants and animals, also some bacteria. “Circadian” comes from the Latin “circa,” (around), and “diem” (day), literally, approximately one day. Circadian rhythms allow our bodies to prepare for environmental changes in advance.

To distinguish circadian rhythms (internal) from the coincidental (external), three criteria must be met:

1. The rhythm must maintain itself over a range of temperatures.

2. The rhythm is able to be reset by an external stimulus. This ability is necessary to allow us to gauge local time when, for instance, we travel across time zones. Resetting our internal clock requires a little adjustment, the adjustment period is known as jet lag.

3. The rhythm must maintain itself in constant conditions, such as total darkness or daylight

At the cellular level, cells interface with the brain to release hormones. These hormones regulate the internal clocks of our organs. Information about the time of day is relayed by the eyes to the internal clock in the brain, thus synchronizing all body clocks. In this way the timing of our sleep/wake cycle, thirst, appetite and body temperature are controlled.

Circadian rhythm sleep disorders affect the timing of sleep. Sufferers are unable to sleep and wake at times required for carrying out their daily routines such as school, work, and social activities. Their body clocks dictate their sleep/wake cycles; if they are allowed to sleep and wake in rhythm with their disordered body clocks, they can get enough “normal quality” sleep. The body may be ready to sleep and awaken at various times of the day.

There are two types of sleep disorders: The extrinsic type, which includes jet lag, and shift work sleep disorder, and the intrinsic type, which includes:

Advanced sleep phase syndrome (DSPS) causes difficulty staying asleep in the morning and staying awake in the evening.

Delayed sleep phase syndrome (DSPS) causes peak alertness in the middle of the night and a late timing of sleep onset.

Irregular sleep-wake pattern causes sleepiness and actual sleeping at irregular times, usually more than once per day; total time asleep is typical for the sufferer’s age.

People who do not suffer from circadian rhythm sleep disorders may fall into certain groups. They may be called “early birds” if they like to go to sleep early and wake early, or “night owls” if they prefer to go to sleep late and get up late. These “normal” people:

Can wake in time for regular activities, and to compensate, go to sleep earlier, if necessary.

After a few days, will become accustomed to the new routine, and awaken feeling refreshed.

Can go to sleep, and awaken at the same time every day, if they want to.

However, people with sleep disorders are not able to easily reset their body clocks. Some do maintain a 24-hour internal cycle, but their hours may be highly irregular; for example, they may not go to sleep until 3:00 a.m. and sleep until noon.