Occasionally I get out of bed in the middle of night and wander around the room, or even around the flat, while remaining fast asleep. And it is not just me: one in five children sleepwalks regularly and at least 40% have done it at least once. As we get older it becomes rarer, but 1-2.5% of adults still do it.

Some sleepwalkers imagine they are escaping from something frightening. Others calmly and methodically look through drawers and cupboards as though searching for something. When I was a child I used to sleepwalk my way downstairs and join my parents watching TV until they led me back up to bed. Anyone who has encountered a sleepwalker will know that it appears to be a very strange state, a state which has features of both sleep and waking.

One piece of advice about sleepwalking which many will have heard is that it is dangerous to wake a sleepwalker, and that waking them up will somehow harm them. This is not strictly true, but there is no doubt that if you do wake them it will not be pleasant for the sleepwalker.

Sleep states

We do not know precisely why the brain issues orders to sleepwalk, but we do know a lot about what happens when we sleep. During the night we cycle through various stages, beginning with a period of light sleep that deepens considerably within twenty minutes or so, and then becomes a bit lighter again before we slip into a phase of rapid eye movement, or REM, sleep. This cycle then repeats several times throughout the night, with the duration of REM sleep extending each time, to the point that by the morning this part of the cycle forms the main part of your slumber.

The time when you are most likely to dream is during REM sleep. During this stage you are paralysed to prevent you from acting out your dreams, but it is during the much deeper sleep that sleepwalking takes place. It is a curious, paradoxical state. The brain is active enough for you to move, but not so active that you wake up. A recent study conducted at Niguarda Hospital in Milan, Italy examined the brain waves of people prone to sleepwalking and found that some parts of the brain are awake, while others are sound asleep, suggesting sleepwalking is caused by an imbalance between these two states.

It is a myth that people walk with their arms out straight ahead like a zombie, but they do tend to have a glazed expression, as though the eyes are unseeing, and it is very hard to get their attention. People do not tend to switch the light on, but navigate around their homes from memory.

In danger?

It is also a myth that you cannot hurt yourself while sleepwalking – you can still trip up and it is when sleepwalkers stray somewhere unfamiliar that they can find themselves in danger, with a good example being if they wander out of the front door and on to the street.

Professor Matthew Walker from University College London Hospital’s Sleep Clinic once told me that he had a patient who went out of his house, got into his car and drove, all while fast asleep. Then there was the case of the 15 year old girl who in 2005 was found curled up asleep at the top of a 130ft crane, having climbed there while sleepwalking.

These cases are rare. Occasional sleepwalking does not tend to present a problem and most children grow out if it. If it happens every night and causes problems, then sleep disorder specialists suggest that parents spend a week noting the time when the sleepwalking tends to happen, and then gently wake the child about fifteen minutes before this time. This will often break the cycle.

So what should you do if you find your relative sleepwalking?

For a start, they are in such a deep sleep that they will probably not notice you, even if you try to wake them. If you do succeed in waking them you might disorientate them to the extent that they become distressed. Everyone has experienced that feeling of utter bewilderment when the alarm awakens you from deep sleep, instead of the lighter sleep we are usually experiencing by the time our alarms sound. I once had the shock of being woken by a crash and found myself in the kitchen, standing barefoot and surrounded by broken glass. I’d tried to get a glass of water while sleepwalking, but like most sleepwalkers I failed to turn on the light, and had smashed the glass against the tap.

So waking a sleepwalker will not cause them to have a heart attack or put them into a coma, but the kindest thing to do is not to try to wake them at all. Lead them gently back to bed so that they do not hurt themselves. They will remain deeply asleep, and it is likely that they will not remember a thing in the morning.

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