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.

Sleep is a highly complex phenomenon in which changes in sensory, muscular, hormonal and neural systems must be coordinated to create a particular state of vigilance. "When you think about it, there are billions of people in the world who are shifting between wakefulness, REM and non-REM multiple times every 24 hours," says Mahowald. "It's amazing that the switching is as accurate as it is." This accuracy suggests that there is a strong adaptive drive to be fully in one state or another. But he als

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