Sleepiness is not always perceived before falling asleep in healthy, sleep-deprived subjects

To test whether subjects spontaneously signal sleepiness before falling asleep under monotonous conditions.

Twenty-eight healthy students were deprived of sleep for one night and then underwent a "maintenance-of-wakefulness test" (MWT) consisting of four 40-min trials. They were told to give a signal as soon as they felt sleepy and to try to stay awake as long as possible. In a first series of tests, the subjects were given no reward (nr); in a second series, monetary rewards (wr) were given both for an accurate perception of sleepiness and for staying awake longer.

Seventeen of the 28 subjects (60.7%) did not signal sleepiness before a sleep fragment occurred in at least one of the four MWT trials. Women were more reliably aware of sleepiness than men in the nr trials (p=.02), while the men's performance improved in the wr trials (p<.02), becoming equivalent to the women's performance.

Our results cast doubt on the general assumption that one cannot fall asleep without feeling sleepy first. If similar results can be obtained in monotonous driving or working situations, this will imply that accidents caused by sleepiness or by falling asleep cannot necessarily be attributed to an individual's negligence.