The exact incidence and prevalence rates of circadian rhythm sleep disorders are not known. One fourth of all chronic sleep disorders are the result of a mismatch between the body’s internal clock and the external 24-hour schedule.

Women suffer from of circadian rhythm sleep disorders more often than men and this is estimated to be twice to three times more common in them. Girls in their teen are more prone. Women’s body clocks speed up sooner than men, usually when in their early 40’s. Most men’s clocks don’t start speeding up or advancing until they reach their 60’s.

Delayed Circadian Rhythm Disorder (DCR) is common. Approximately 7-10% of patients who complain of insomnia are diagnosed with a circadian rhythm disorder, most often DCR. The prevalence of DCR is probably higher than that because the total sleep time is typically normal in patients with DCR and because patients with DCR adjust their lifestyle to accommodate their sleep schedule and do not seek medical treatment. In adolescence, the prevalence is approximately 7%.

Advanced Circadian Rhythm Disorder (ACR) probably is quite rare. However, an age-related phase advance is common in the elderly, who tend to go to sleep early and get up early.

The prevalence of Bimodal Circadian Rhythm Disorder (BCR) or Irregular Sleep/Wake Cycle has not been established but is said to be quite high. Irregular sleep-wake schedule is common in patients with Alzheimer disease.