There is no truth in the old wives' tale that sleep before midnight is more beneficial than sleep after midnight: what counts is getting enough sleep at the same time every night.
That "jump", or sensation of falling, which sometimes wakes you with a jolt is really a hypnagogic hallucination as your skeletal muscles relax on the way to slow-wave sleep.
REM sleep isn't just about dreaming: genital arousal during this stage of sleep is quite normal for both sexes and occurs in about 95 per cent of REM cycles.
There really is such a thing as a biological clock: two tiny neural structures, the suprachiasmatic nuclei, in the centre of the brain, control rhythms of alertness, body temperature and hormone production.
The official record for going without sleep was set by one Randy Gardner in 1965. He stayed awake for 264 hours and 12 minutes - then slept for 14 hours, 40 minutes.
Yawning (the word is derived from the old English ganien, meaning to gape or open wide) is a definite sign of sleepiness. It's also contagious. What it isn't, is a response to an excess of carbon dioxide or a shortage of oxygen.
Up to 90 per cent of people grind their teeth during sleep: five per cent do so chronically, a condition known as `bruxism".
Everyone has an "alertness trough" between 2 and 4 p.m every day.