What is Non-24 Hour Sleep Wake Disorder?
Non-24 Hour Sleep Wake Disorder (N24HSWD) is a disorder that affects the normal 24-hour synchronization of circadian rhythms. One subtype designation according to the International Classification of Sleep Disorders is Circadian Rhythm Sleep Disorder - Free Running Type. It has also been called Hypernychthemeral Syndrome.
All animals and plants have an endogenously generated near-24-hour 'circadian' clock rhythm (that is, an "internal body clock") that regulates the approximate 24-hour cycle of biological processes, such as sleep or hormone production. These rhythms have a period close to, but not exactly 24 hours, and are synchronized daily by environmental time cues. In mammals, circadian rhythms are generated by the suprachiasmatic nuclei (SCN) in a structure of the brain known as the hypothalamus with the day-night cycle as the primary environmental time cue that synchronizes the circadian system to the 24-hour day. Many people are unaware that their ability to sleep at night and be awake in the day is largely controlled by their internal body clock where light is the primary cue to help reset the internal body clock.
People with N24HSWD have circadian rhythms that are not synchronized with the 24-hour day-night cycle, either through a failure of light to reach the SCN, as in total blindness, or due to various other reasons in sighted people. People have internal body clocks that are slightly longer than 24-hours. Daily environmental cues, such as light, resets people's circadian period back to the 24-hour day-night cycle. For example, if someone was on a 24.5 hour clock, they would sleep 30 minutes later on the first day, then one hour later on the second day, and so on. For someone with a longer circadian delay (i.e., on a 25-hour clock rather than 24.5-hour clock) sleep disturbance and departure from the 24-hour light-dark cycle surfaces much quicker. Consequently, sleeping at night becomes more difficult and the drive to sleep during the day increases. Eventually, the person’s sleep-wake cycle realigns with the 24-hour light-dark cycle and they are able to enjoy the conventional sleep period once again. However, this period of good sleep is only temporary as the sleep cycle continues to shift later.
The time it takes to complete one circadian cycle depends on the timing of the internal clock; in an individual with a 24.5 h clock, it will take 49 calendar days to complete a full circadian cycle and might have 4-5 weeks of poor sleep per cycle, whereas someone with a 24.1 h clock will take 241 days to cycle and might have 4-5 months of poor sleep each cycle.
The majority of blind people have some light perception and circadian rhythms that are synchronized to a 24-hour day-night cycle as in the sighted. For a totally blind individual with N24HSWS, their visual disorder or lack of eyes prevents the light-dark cycle from synchronizing their internal body clock to the 24-hour day-night cycle. Often, the sleep disturbance is less clear, with more subtle changes in the timing of sleep, or may even look normal, even though the circadian clock is still not synchronized with the 24-hour cycle. Only an assessment of a strong biochemical circadian rhythm, such as the melatonin or cortisol rhythm, can confirm whether a non-24-hour rhythm is absent or present.
Am I alone in having Non-24 Hour Sleep Wake Disorder?
N24HSWD is common in totally blind people due to the lack of light information received from the eyes, which normally regulates the 24-hour day-night cycle. As a result, in the totally blind, the internal body clock reverts to a non-24 hour period, causing fluctuating periods of good sleep followed by periods of poor sleep and excessive daytime sleepiness. It has been estimated that of the 1.3 million blind people in the United States, 10% have no light perception at all. N24HSWD is most common in totally blind individuals. Reports suggest that as many as half to three-quarters of totally blind patients have N24HSWD, representing approximately 65 to 95 thousand Americans. The first signs of the disorder can occur at any age and usually happens shortly after complete loss of light perception or surgical removal of the eyes.