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

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