and attempted murder of his parents-in-law. One night in the previous year, he drove 23 kilometres across town to their home and attacked them with a tyre iron and a knife. He was acquitted on all counts, on the grounds that he was sleepwalking at the time. Since then, numerous defendants have pleaded "parasomnias" - an old-fashioned term for disorders involving partial arousal during sleep that is now mostly used in law. Mark Pressman, at the Lankenau Hospital in Wynnewood, Pennsylvania, estimates that they are used as a defence in between five and 10 cases worldwide each year.

This raises tough issues about criminal responsibility and has been a source of ongoing dispute among lawyers. One of the most contentious areas concerns men charged with sexual assault who claim sleep-related sexual behaviour, or sexsomnia, as a defence (New Scientist, 28 October 2006, p 40). Sexsomnia is a variation on sleepwalking and a recognised condition. The controversy arises when these men claim - sometimes successfully - that heavy alcohol consumption provoked their sexsomnia. "My colleagues and I have been actively opposing this defence on the grounds there is no scientific evidence that severe intoxication causes sleepwalking," Pressman says. But he admits that it is a fiendishly difficult area of law, not least because no sleep study can shed light on what happened in a single individual's brain on a single night in the past.

Ground control to Major Tom

Every so often, commercial pilots and other professionals whose jobs require them to be highly vigilant have to take something called the maintenance of wakefulness test. They must sit in a comfy chair in a dimly lit room and stay awake for 40 minutes, a procedure that is repeated four times over the course of 8 hours. If they should drop off, this is detected by EEG, eye movement and jaw slackness.

However, these are fairly crude measures, and anyway people whose job is on the line tend to be highly motivated to stay awake. Sleep scientists acknowledge that a more sensitive test is required, and David Dinges at the University of Pennsylvania, Philadelphia, believes he has one.

His is a test of reaction time in which subjects have to repeatedly press a button in response to a light going on. This psychomotor vigilance test is "unbelievably sensitive" to instability in the boundaries between wakefulness and sleep, Dinges claims. A shortened version went up to the International Space Station in July, so that the astronauts could put it through its paces. If it accurately reflects their ability to do their job, it may soon be routinely applied to pilots, nuclear power station operators and others in life-or-death jobs.

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