o thinks that state dissociation - the presence of more than one vigilance state concurrently - is more common than anyone previously suspected.

The boundaries between sleep and wakefulness are particularly blurred when we are sleep-deprived. Around a decade ago, Dinges realised that although his sleep-deprived volunteers seemed to be awake, they were in fact experiencing momentary lapses, or microsleeps. Since then, he has discovered that these fleeting naps last between half a second and 2 seconds, and become increasingly frequent the longer we are deprived of sleep, until finally we cannot recover and nod off. Dinges sees them as the outward sign of a tug-of-war between neural systems that are trying to initiate sleep, and others that are trying to maintain wakefulness.

This chimes with the ideas of James Krueger of Washington State University in Pullman, who has argued that the individual processing units in the brain - known as cortical columns - fall asleep independently when they become tired. In his view, shifts between wakefulness and sleep occur when enough columns are in one state or the other (Nature Reviews Neuroscience, vol 9, p 910). Krueger believes this mosaic pattern of sleep explains sleep inertia and sleepwalking.

Some people are more prone to microsleeps than others. In a 2007 study, Dinges and his colleagues showed that there are enormous differences in people's ability to resist the lure of sleep when tired. Among a group of healthy, non-sleep-deprived adults, the differences in alertness are small. "But as soon as we provoke the system with some sleep deprivation, those differences get larger and larger," he says (Journal of Sleep Research, vol 16, p 170).

This has implications for all of us, especially those in professions where staying awake is a life-or-death matter (see "Ground control to Major Tom"). "If you're doing 100 kilometres per hour on a highway and you have a lapse, your fingers go lax on the steering wheel and you drift off the road at a 4-degree angle," says Dinges. "Two seconds is all it takes to be completely out of your lane." Up to 20 per cent of traffic accidents are fatigue-related.

Brain imaging has recently revealed a mental back-up system in people who remain alert when sleep-deprived (Journal of Neuroscience, vol 29, p 7948). While other people have reduced brain activity when tired, sleep-resistant individuals manage to maintain their brain activity levels. More interestingly, they also recruit new areas to help compensate for having been awake for so long. These people were selected for the study because they had a gene variant found in around 40 per cent of people that is thought to be associated with resistance to sleep deprivation. It seems likely that such people are also less prone to state dissociation, although this has not yet been tested.

上一页  [1] [2] [3] [4] [5] 下一页