Non-24-hour Sleep-wake Disorder

Non-24-hour sleep-wake disorder (Non-24) is a chronic circadian rhythm sleep disorder, classified within Chapter VI, Diseases of the Nervous System, in the ICD-10. It can be defined as "a chronic steady pattern comprising one- to two-hour daily delays in sleep onset and wake times in an individual living in society". The pattern of delay persists literally "around the clock", typically taking a few weeks to complete one cycle. This disruption of the body clock causes cyclical bouts of nighttime sleeplessness and excessive daytime fatigue and napping. People with Non-24 "resemble free-running, normal individuals living in a time-isolation facility with no external time cues".

Non-24 is a neurological sleep disorder. It has been traced to a number of genetic abnormalities that affect specific biologic factors in the brain. While both sighted and blind people are diagnosed with Non-24, the disorder affects proportionately more totally blind individuals than sighted.

Though often referred to as Non-24, it is also known by the following terms:

• Free running disorder (FRD)
• Hypernychthemeral disorder
• Circadian rhythm sleep disorder – free-running type
• Circadian rhythm sleep disorder – nonentrained type
• Non-24-hour circadian rhythm disorder
• Non-24-hour sleep-wake disorder

The disorder is an invisible disability and "is extremely debilitating in that it is incompatible with most social and professional obligations".