Are you asleep? Exploring the mind's twilight zone

EARLIER this year, a puzzling report appeared in the journal Sleep Medicine. It described two Italian people who never truly slept. They might lie down and close their eyes, but read-outs of brain activity showed none of the normal patterns associated with sleep. Their behaviour was pretty odd, too. Though largely unaware of their surroundings during these rest periods, they would walk around, yell, tremble violently and their hearts would race. The remainder of the time they were conscious and aware but prone to powerful, dream-like hallucinations.

Both had been diagnosed with a neurodegenerative disorder called multiple system atrophy. According to the report's authors, Roberto Vetrugno and colleagues from the University of Bologna, Italy, the disease had damaged the pair's brains to such an extent that they had entered status dissociatus, a kind of twilight zone in which the boundaries between sleep and wakefulness completely break down (Sleep Medicine, vol 10, p 247).

That this can happen contradicts the way we usually think about sleep, but it came as no surprise to Mark Mahowald, medical director of the Minnesota Regional Sleep Disorders Center in Minneapolis, who has long contested the dogma that sleep and wakefulness are discrete and distinct states. "There is now overwhelming evidence that the primary states of being are not mutually exclusive," he says. The blurring of sleep and wakefulness is very clear in status dissociatus, but he believes it can happen to us all. If he is right, we will have to rethink our understanding of what sleep is and what it is for. Maybe wakefulness is not the all-or-nothing phenomenon we thought it was either.

Received wisdom has it that at any given time, healthy people are in one of three states of vigilance: awake, in rapid eye movement (REM) sleep or in non-REM (NREM) sleep. Each state is distinct and can be recognised by a characteristic pattern of brain activity, as measured by an electroencephalogram (EEG) (see chart, right). Wakefulness is easy to detect. Apart from the fact that a person's eyes are open and they are responsive, their EEG shows a pattern of high-frequency, low-amplitude waves. NREM sleep is divided into four stages, each of which has its own distinctive EEG pattern. REM is trickier to spot because in EEG terms it closely resembles stage 1 NREM sleep. So to be sure it really is REM, researchers also look for the telltale rapid eye movements and a slackening in the muscles of the chin and jaw.

Mahowald is not the only person to have questioned these neat distinctions. David Dinges, a psychiatrist at the University of Pennsylvania, Philadelphia, has probably deprived more people of sleep in the name of science than anyone else. In one such study in the late 1980s, Dinges and his team revealed how easily the different states of vigilance can become intermingled. When volunteers were subjected to tests of working memory in which they had to subtract numbers, they could do an average of 90 sums in 3 minutes with few errors. After 52 hours deprived of sleep, their performance fell to around 70 subtractions, with not many more errors. However, after they had slept for 2 hours the change was dramatic. "When we woke them up abruptly, and they rated themselves as alert and ready to go, they couldn't do even one subtraction," says Dinges. People even seemed to be dreaming as they attempted the task. One subject mused: "What if people ran faster than normal people run home," in the middle of a string of incorrect responses.

Known as sleep inertia, a less extreme version of such disorientation is now generally recognised as the cause of the grogginess some people get after their alarm clock goes off. It is as if they are socially awake but functionally asleep; as if the brain circuits underlying responsiveness are up and running, but those mediating working memory are still offline. Mahowald is convinced that it is just one of many disorders that can be explained as a breakdown in the boundaries between sleep and wakefulness. He lists a whole raft of such conditions in the same issue of Sleep Medicine as Vetrugno's description of people with status dissociatus (vol 10, p 159).

One is REM behavioural disorder (RBD), in which people in REM sleep act out their dreams because the temporary paralysis, or cataplexy, that normally accompanies this state is replaced by the full mobility of wakefulness. In sleep paralysis the opposite is true. Here, cataplexy intrudes into wakefulness, and a person wakes to find him or herself unable to move. It is estimated that up to 40 per cent of people have experienced this disturbing phenomenon. Also surprisingly common are hypnagogic hallucinations - sensory illusions that occur on the cusp of sleep when the dreaming component of REM intrudes into wakefulness. Mahowald's list also includes sleepwalking, night terrors and narcolepsy, which is an inherent instability in vigilance state boundaries characterised by rapid cycling between states and the tendency to fall asleep mid-sentence. Controversially, the list also includes near-death experiences and alien abductions. It is no coincidence, he says, that alien abductions almost always occur in the recumbent position, in the transition from wakefulness to sleep.

via Are you asleep? Exploring the mind's twilight zone - life - 07 October 2009 - New Scientist.