Contributor Lynne Lamberg – 5-part Series: Improving Sleep and Alertness in the Blind

Part 2: How the body clock works

Non-24-hour sleep-wake disorder (N24HSWD) is a little recognized consequence of living in constant darkness.

In sighted people, daylight exposure organizes daily life. It sets the hands of the body’s master clock, a tiny pair of nerve clusters in the center of the brain that anchor body rhythms to the earth’s 24-hour light/dark cycle.

This master clock governs the alternation of sleeping and waking, body temperature, blood pressure, secretion of hormones, hunger, urination, cell division, mood, and hundreds of other physical and mental functions that occur in synchrony with the earth’s 24-hour rotation. These daily rhythms are called “circadian,” from the Latin words, “circa,” meaning about, and “dies,” a day.

Humans and most other day-active species have a circadian clock that usually runs on a slightly longer than 24-hour cycle, typically, 24 to 25 hours long. This built-in flexibility allows people to stay up late or get up early if desired, travel across time zones, live at different latitudes, and adapt to changing day length over the year.

Researchers study circadian clocks in caves and windowless research laboratories with no clocks or other indicators of time. Both sighted and blind people participating in such studies typically go to sleep and get up from a few minutes later to about an hour later day after day. Each person’s schedule is unique.

In the outside world, daylight keeps internal clocks in sighted people from drifting. Daylight synchronizes inner rhythms with the planetary light/dark cycle every day.

The eyes, and only the eyes, detect light for this purpose, Lockley said. Light signals travel from the eyes to the master clock over a special pathway that differs from the one used to send visual signals.

About 55 per cent of people who totally lack light perception, and virtually all of those with no eyes, experience N24HSWD, Lockley said. A small proportion of blind people with no conscious awareness of light perception still retain the ability to detect light, which enables circadian synchronization of their body rhythms. Visually impaired people who can detect light seldom develop N24HSWD.

In 1977, Laughton Miles and his colleagues at Stanford University published a landmark report, “Blind man living in normal society has circadian rhythms of 24.9 hours.” Despite the man’s diligent efforts to stick to a regular schedule for bedtime, rise time, work, and meals, his body rhythms continued to drift around the clock.

A person whose body clock runs on a 25-hour cycle may go to sleep an hour later each day, moving around the clock in 25 days. He or she may experience about two weeks of good sleep followed by two weeks of bad sleep, and then repeat this cycle.

Someone whose body clock runs on a 24.1 hour schedule, however, will drift only six minutes per day and take 241 days, or about eight months, to go around the clock. This person may sleep well for four months, and then badly for four months, before returning to good sleep, then bad sleep, and so on. Such individuals and their physicians may attribute the sleep disturbance to ordinary insomnia rather than to a body clock disorder. Ordinary insomnia, however, does not wax and wane with such regularity.

Humans occasionally have body rhythms that are shorter than 24 hours. These people go to sleep earlier and get up earlier each day, drifting backwards around the clock. Totally blind people who report no cyclic difficulties may have an internal clock with a period very close to 24 hours that is synchronized by time cues other than light, such as a regular schedule for sleep, work, meals, and exercise. Some people have a clock that runs on a 24-hour cycle.

Part 3 of this report will describe efforts to help people with N24HSWD stabilize their internal clocks.

Link to Introduction from the Editor:

Link to Part 1:

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