Circadian Rhythm Sleep Disorders are a group of sleep disorders that affect the timing of sleep, characterised by an inability to sleep and/or wake at normal or appropriate times due to the dictates of the individual’s biological or circadian clock.

People with circadian rhythm sleep disorders are therefore unable to sleep and wake at the times required for normal work, school, and social needs, which can severely impact their quality of life.Circadian Rhythm Sleep Disorders include:

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Advanced Sleep-Wake Phase Syndrome - ASPS
Advanced sleep-wake phase syndrome involves a shift in the circadian rhythm that leads to a strong, sometimes irresistible need to fall asleep in the early evening, generally between 6 p.m. and 9 p.m., and wake up very early in the morning, generally between 2 a.m. and 5 a.m.

Delayed Sleep Phase Syndrome - DSPS
Delayed Sleep Phase Syndrome involves a shift in the circadian rhythm that is opposite to ASPS. People with DSPS are unable to fall asleep until very late at night and not wake up until much later in the day.

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Irregular sleep–wake rhythm

This is a rare form of circadian rhythm sleep disorder. It is characterised by numerous naps throughout a 24-hour period, no one main sleep episode that occurs at any time of day and irregularity from day to day. During the day, it may seem like they are sleepy because they nap so much. During the night, it may seem like they have insomnia because they are awake for long periods of time during the night. The total time asleep per 24 hours is normal for the person's age.

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Non-24-hour Sleep-Wake Disorder 
The sleep time of people who have Non-24-Hour Sleep-Wake Disorder shifts a little later every day. Sleep time and wake up time continue to move later and later every day. Every day, morning light and other behaviours reset the sleep-wake clock to a 24 hour schedule. Without light or if there is a fault with this clock resetting, people’s sleep time will drift later and later. This is why many people who have Non-24-Hour Sleep-Wake Disorder are blind.