Circadian rhythm sleep disorders happen when the circadian rhythm is not working correctly. They make people sleep and wake at times that are not normal, but these people usually have 8 hours of good sleep. These disorders affect other systems in the body, too, and not just sleep. Circadian rhythms also control appetite, body temperature rhythm and hormone cycles.

Extrinsic category 
"Extrinsic" means something which comes from outside rather than from within a system or an organism. There are two circadian rhythm sleep disorders of the extrinsic type:

 Jet Lag
Jet Lag often affects people who have travelled several hours east or west, through several time zones. It may take many days for the body to adjust. The name "jet lag" comes from travelling by jet aircraft.

 Shift work syndrome
Many people have health problems when they work a shift during the times when their circadian rhythms say that they should be asleep.  Most people who work later than midnight find it difficult to stay awake.

Intrinsic category 
"Intrinsic" means something which happens or comes from within a system or an organism. There are four circadian rhythm sleep disorders of the intrinsic type. Unless they are caused by a head injury, they are usually genetic, that is, people are born with them. They are generally chronic; they do not go away, but they may sometimes be treated.

 Advanced sleep-phase syndrome
Advanced sleep-phase syndrome (ASPS) is very rare. People with this syndrome are sleepy very early in the evening and they sleep from about 19:00-20:00 in the evening to about 03:00-04:00 in the early morning. Often several people in the same family have ASPS.

 Delayed sleep-phase syndrome
Delayed sleep-phase syndrome (DSPS) is more common than ASPS and it affects about 0.15% of adults. It usually starts in early childhood or at puberty. Some cases that start at puberty go away when the person is about 20 or 21 years old. DSPS is much more common in teenagers than in adults. DSPS in people older than about 20 usually will have it all their lives, that is, it is chronic.

People with DSPS cannot go to sleep before 03:00-06:00 in the morning, or even later. They cannot wake up in time for school or a job which starts in the morning. They can go to sleep at the same time every night and get up at the same time each day. This means that they are entrained (adjusted, synchronized) to the daily light/dark cycle. Even when they try to sleep at "normal" times, they feel most awake late at night.

 Non-24-hour sleep-wake syndrome
Non-24-hour sleep-wake syndrome (Non-24) is quite rare. People with Non-24 have very unusual sleep timing. Their sleep and wake times are not regulated by daylight; they are not entrained (adjusted, synchronized) to the daily light/dark cycle. This is called freerunning sleep.

The worst circadian rhythm sleep disorder in people who can see is Non-24; their "daily" rhythm lasts 25 hours or more so they sleep and wake up one or more hours later each day. Appetite, body temperature rhythm and hormone cycles all keep moving around the clock, coming back to the same time after a few weeks.

About half of totally blind people, who can see no light at all, have Non-24. Their daily rhythm is not exactly 24 hours and it does not adjust to the light/dark cycle in nature because they can not see light. This can often be treated.

 Irregular sleep-wake disorder
Irregular sleep-wake disorder is rare, and it is often possible to find out what causes it, for example, a head injury, a mental handicap, dementia or some other sicknesses. People with this disorder sleep and wake several times a day, but not at the same times every day. They sleep about 8 hours a day but never 8 hours together.

Treatment 
Treatment for these disorders often does not work well. In addition, it can be difficult to find a doctor who knows much about circadian rhythms, as the field of study is quite new.

 Bright light
Treatment with bright light is also called phototherapy or light therapy. People with ASPS need a lot of light in the evening and they should avoid bright light in the morning. People with DSPS need bright light as soon as they wake up and should try to avoid bright light before bedtime.

 Melatonin
Melatonin is a hormone produced in the pineal gland in the brain. It is normally only produced in the evening and at night. People with ASPS can take melatonin by mouth in the morning. People with DSPS can take melatonin in the afternoon or evening, and this treatment works also for some people with Non-24.

One can buy melatonin in the shops in the USA and Canada. In most other countries one must get it from a doctor or it might not be legal at all.

Sleep disorders
Disability