Why do some of us do bizarre things in our sleep? Like riding a motorbike, using a shoe to 'phone for a pizza or even having sex while sleeping? These are complex behaviours and yet sleepwalkers aren't aware of what they're doing and often have no memory of their strange night-time activities.

These sleep disorders are known as non-REM parasomnias and include conditions like night terrors and sleep eating.

So why does it happen? Sleepwalking usually occurs during deep sleep, when something triggers the brain to wake - but not completely. So the areas that control walking and other movement wake up, yet other parts, involved in awareness and rational thinking, remain asleep. What's confusing is that sleepwalkers look awake - their eyes are open - but they're really not awake. They're not really asleep either. The brain is awake and asleep at the same time. We have known this happens in some animals, who can sleep with half of their brain at a time. But recently, we have learnt that similar things can happen in the human brain.

In the first of a three-part series, neurologist, Dr Guy Leschziner, talks to patients he's been treating at his sleep clinic at Guy's and St Thomas' hospitals in London. They include Jackie who began sleepwalking as a child and continued her strange night-time behaviour as an adult, riding her motorbike whilst sleeping. We hear from James whose night terrors have become so violent his wife has begged him to get help; from Alex who rescues people from floods in his sleep. And we talk to Tom, whose recent diagnosis of sexsomnia has had a significant impact on his life.

These remarkable sleepwalking experiences help us to understand the complex workings of the human brain.

Presenter: Dr Guy Leschziner Producer: Sally Abrahams.