Some people have an irregular sleep-wake pattern because of a problem with brain function, the body's internal clock (circadian pacemaker), or other reasons.
This disorder is very rare. It usually occurs in a person who has a problem with brain function and who does not have a regular routine during the day. The amount of total sleep time is normal, but the body clock loses its normal circadian cycle.
People with changing work shifts and travelers who often change time zones may also have these symptoms. These people have a different condition, such as shift work sleep disorder or jet lag syndrome.