Another group who appear to be more vigilant than most are people who suffer from primary insomnia - insomnia not associated with any other condition. There is evidence to suggest they are in a constant state of hyperarousal, with relatively high metabolic rates and high levels of the stress hormone cortisol. "It's as though somebody just cranked up their alertness over 24 hours, so they are more alert at night, but they are also more alert in the day," says Mahowald.

As the blur between sleep and wakefulness becomes more widely accepted, researchers are devising techniques for capturing the brain's fleeting lapses and vacillations. For example, neuroscientist Giulio Tononi of the University of Wisconsin, Madison, is eavesdropping on sleeping brains using EEGs with 256 electrodes rather than the more usual 32, to improve spatial resolution and help him catch the brain in the act of mosaic napping. Microsleeps are just the tip of the iceberg, Tononi says. He is particularly concerned by the possibility that parts of our brain might be going offline without us even realising it. "In many respects, it would be like having a temporary mental disorder without anybody, including yourself, being aware of it," he says. Forgetfulness and daydreaming could be examples of this, but so could more bizarre and even criminal behaviours (see "When a crime is not a crime").
It is like having a temporary mental disorder without anyone, including yourself, being aware of it

Meanwhile, Pierre Maquet at the University of Liège in Belgium has started to use fMRI to chart the patterns of brain activity associated with different sleep states. His team is already finding that the distinctions between sleep and wakefulness look quite different when you compare patterns of activity across the whole brain, including deep structures, as opposed to using EEG, which measures activity in just the outer few millimetres of the cortex.

Lurking in the background is the hope that these approaches will shed light on the vexed issue of what sleep is for. A leading theory is that it is important for memory consolidation. Yet one of the puzzling aspects of the two Italians with status dissociatus is that, despite complete disruption of both their REM and NREM sleep, they showed no memory deficit. Does this add grist to the mill of those who believe that sleep has no other purpose than to save energy and keep us safe (New Scientist, 15 March 2008, p 30)? Or does it mean, as Mahowald believes, that the two individuals were in fact experiencing some sort of mosaic sleep? Armed with a less black-and-white definition of sleep and wakefulness, and more sensitive tools for measuring them, that question might finally be resolved.

When a crime is not a crime

In 1988, a 23-year-old Canadian man named Kenneth Parks went on trial for the murder

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