Sleep and blindness.

Normally, one's sleep-wake cycle (i.e., circadian rhythm) is strongly coupled to the rise and fall of light throughout the day. Like a silent alarm, the slowly increasing brightness of daybreak gently awakens a person. The full strength of daylight at noon is a signal for many to take a short break in one's labors. The slowly diminishing light of early evening signals the time to quit work for the day. The disappearance light at dusk works like a lullaby to bring on sleep. Since blind people live without a dependence on light for every day living, scientists long believed that their circadian rhythm was not coupled with daylight and darkness. However, scientists have found in recent years that some blind people maintain a circadian rhythm that is coupled to natural daylight and darkness. This and other recent findings are causing scientists to focus on the unique aspects of sleep in blind people.

Laughton E. Miles was the first scientist to report a free-running rhythm in a blind person in 1977. His subject was a blind man who suffered from insomnia at night and sleepiness during the day. The man noted that onset of his daytime sleepiness occurred one hour later each day. This kind of delay meant that the man was suffering from a free-running rhythm. Additionally, Miles found that the man's biorhythms (e.g., temperature and cortisol secretion) were on a circadian rhythm of nearly 25 hours rather than 24 hours.

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After Miles' study, many studies on sleep in blind people were done with the intent of understanding the sleep of sighted people. In other words, the focus of studies was not on sleep problems unique to blind people. Although some of these studies noted a free-running rhythm in some of their subjects, the prevalence varied from study to study. For example, Lewy and Newsome2 found that only one of their 10 subjects suffered from a free-running rhythm; Miles and Wilson3 found that 35% of their subjects had a free-running rhythm; and Sack et al.4 found that about 50% of their subjects had a free-running rhythm.

Since these studies used small subject groups ranging from 10 to 50 subjects, Damian Leger et al.5 compared the sleep of 1073 blind people with the sleep of sighted people (i.e., the control group) to get a better idea of the types of sleep disorders found most frequently in blind people. The subjects answered a 48 item mail-in questionnaire. Questions covered issues such as: sleep habits, sleep disorders, daytime alertness, mood, memory, and type and degree of blindness. The blind subjects were then divided into three groups: (1) subjects who were born blind; (2) subjects who were totally blind (no light perception); and (3) subjects who were almost blind (no vision in one eye and less than 10% vision in the other eye). Leger et al. found;

* 3 times as many of the blind subjects (6.4%) than controls (2.3%) slept 5 hours or less at night.

* The totally blind and almost blind groups when compared to each other had nearly equal rates of difficulty in: falling asleep (totally blind, almost blind--35%, 37% respectively); awakening during sleep (55%, 49%); early morning awakenings (47%, 37%); non-restorative sleep (34%, 37%); and poor sleep quality (48%, 53%). By contrast, an average of 25% of the controls suffered from each of these difficulties.

* The onset of sleep problems began with the loss of vision in 25% of blind subjects. Of those who had been blind from birth, 68% had suffered from sleep problems since childhood.

* Sleep latency was twice as long in blind people compared to controls. Part of the delay was that visual problems made getting ready for bed more difficult. Although most people were asleep within an hour, 8% of the blind subjects reported having a sleep latency of 1 hour or more compared to 2% of the controls.

* Twice as many of the blind subjects found it difficult to go back to sleep after an arousal compared to the controls. Approx. 31.5% of the blind subjects reported awakening 3 times or more during the night compared to 22.5% of the controls.

* The totally blind and almost blind groups used sleeping medication at nearly equal rates: (26% of the totally blind used sleeping medication vs. 20% of the almost blind) while only 13% of the control subjects used sleeping medication.

* About three times as many blind subjects (14.3%) than controls (5.5%) reported involuntary daytime sleep episodes. However, 63.4% of the blind subjects struggled with daytime sleepiness compared to 47.5% of the controls.

* Since diagnosing a free-running rhythm in the subjects would have required measuring biorhythms (e.g., melatonin production), Leger et al. inferred from subject's responses that a free-running rhythm was occurring in 18% of the totally blind subjects, in 13% of the almost blind subjects, and in 8% of the controls.

Blindness has a unique impact on sleep. Blindness increases the amount of time needed to do daily tasks including going to bed and getting ready in the morning; this can decrease the amount of sleep time available. Many blind people complain of increased stress resulting from the need to be constantly on the alert in order to avoid accidents; this state of alertness can remain until bedtime and hinder the onset of sleep. Circadian rhythm disorders are common in blind people; resulting in sleepiness and insomnia.

Scientists have tried various therapies to treat circadian rhythm disorders in blind people. One treatment which shows great promise is melatonin.

Melatonin is a hormone which plays a role in inducing sleep. During sleep, the pineal gland synthesizes melatonin but stops production with the onset of daylight. The waxing and waning of melatonin production normally follows a daily 24 hour rhythm. The melatonin level begins to rise at the onset of dark peaking at its highest about the middle of the night; the melatonin level then begins to fall reaching its lowest point about 12 hours later.

In blind people, melatonin production is desynchronized from this schedule. The most common desynchronization is that of a free-running rhythm. This means that the daily high and low levels of melatonin production continually shifts later and later each day. Another type of desynchronization is that of the melatonin production occurring during the daytime rather than during night time.

In a 2000 study, Alfred Lewy found that 10.0 mg of melatonin was able to stop a free-running rhythm in 6 of 7 blind subjects. Later, he was able to reduce the amount to 0.5 mg in 3 of the subjects. These subjects maintained a normal 24 hour circadian rhythm rather than returning to a free-run.

Interestingly, some blind people are able to maintain a natural circadian rhythm despite being unable to perceive light. Scientists believe that this may happen because the light pathways in the brain remain functional. Czeisler et al.6 used bright light to reduce melatonin production in 11 blind subjects with no light perception and 6 sighted subjects (i.e., controls). Melatonin production decreased in 3 of the 11 blind subjects. Czeisler et al. then had the three cover their eyes while undergoing bright light exposure again. This time, melatonin production did not decrease in 2 of the 3. This meant that their circadian pacemaker still responded to light although consciously they could perceive no light.

These new findings are causing scientists to ask questions that are specific to the sleep of blind people. Learning the reasons behind the success of melatonin with some blind people and how light paths remain functional in some totally blind people may one day help improve the sleep of all blind people with sleep disorders.

by Regina Patrick RPSGT