THAT'S THE WHY:FOR MOST of us, a night's sleep means we stay put, more or less. But not for others. Some people have been reported to carry out quite extraordinary feats in their sleep – from walking around to cleaning the house, driving a car and even carrying out violent attacks. Then on waking, the person has no recollection of it.

So what makes people sleepwalk?

Normally when we sleep, we move through a number of different sleep stages. But in those who sleepwalk, the brain appears not to cycle between stages properly, and the person remains in non-rapid-eye- movement (non-REM) stages of deep, non- dreaming sleep during which they carry out the sleepwalking activity.

Sleepwalking, or "somnambulism", also overcomes our usual inhibition of movement during slumber. Usually, the chemical messenger GABA suppresses motor functions when we sleep, explains neurologist Antonio Oliviero in a Scientific Americanarticle.

But he points out that the neurons that release this inhibitory chemical are not yet fully developed in children, which could explain why sleepwalking occurs more frequently in younger people who can later grow out of it.

There’s most likely a genetic tendency to sleepwalk – if one identical twin sleepwalks it’s likely the other does too – and a number of environmental triggers have also been pinpointed, including sleep deprivation, fever, alcohol, pregnancy, thyroid dysfunction, certain medications and stress.

If you wake a sleepwalker, they will probably be confused and disorientated, but it may be necessary if they are at risk of injuring themselves. Otherwise, if they are steered gently back to bed and sleep, they probably won’t remember any of it.