Sleep

Restless

A strang case raises the
question of what sleep is for

THE function of sleep, according to one school of thought, is to consolidate memory. Yet two Italians have no problems with their memory even though they never sleep. The woman and man, both in their 50s, are in the early stages of a neurodegenerative disease called multiple system atrophy. Their cases raise questions about the purpose of sleep.
Healthy people rotate between three states of vigilance: wakefulness, rapid eye-movement (REM) sleep and non-REM sleep. But all three are mixed together in the Italian patients. The pair were initially diagnosed by Roberto Vetrugno of the University of Bologna and his colleagues as suffering from REM behavioural disorder, in which the paralysis, or cataplexy, that normally prevents sleeping people from acting out their dreams is lost. This can cause people in REM sleep to twitch and groan, sometimes flailing about and injuring their bedmates. These patients, however, soon progressed from this state to an even odder one, according to a report in Sleep Medicine.
One of the principal ways to measure sleep is to monitor brainwave activity, which can be done by placing electrodes on the scalp in a technique known as electroencephalography (EEG). Non-REM sleep itself is divided into four stages defined purely by EEG patterns; the first two are collectively described as light sleep and the last two as deep or slow-wave sleep. When the Italian patients appeared to be asleep, their EEGs suggested that their brains were either simultaneously awake, in REM sleep and non-REM sleep, or switching rapidly between the three. Yet when subjected to a battery of neuropsychological tests, they showed no intellectual decline.

Mark Mahowald of the University of Minnesota Medical School, whose group first described REM behavioural disorder in 1986, thinks memory consolidation is still going on in the brains of the two Italian patients; hence their lack of cognitive impairment or dementia. What needs to be revised in light of their cases, he says, is the definition of sleep.
Dr Mahowald suspects that sleep can occur in the absence of the markers that currently define it, which means those markers are insufficient. What’s more, the Italian cases lend support to an idea that has been gathering steam in recent years: that wakefulness and sleep are not mutually exclusive. In other words, the human brain can be awake and asleep at the same time.
That evidence takes the form of a growing list of conditions in which wakefulness, REM and non-REM sleep appear to be mixed. An example is narcolepsy, in which emotionally laden events trigger sudden cataplexy. When the dreaming element of REM intrudes into wakefulness, which can happen with sleep-deprivation, the result is wakeful dreaming or hallucinations. Since such dreams can be highly compelling, Dr Mahowald thinks they might account for some reports of alien abduction.
But there is another possible explanation of the Italian puzzle: that sleep is not necessary for memory after all. Jerry Siegel of the University of California, Los Angeles, has studied the sleep habits of many animals and thinks that could well be the explanation. All of which gives researchers something new to keep them awake at night.