Anybody can have a night terror but they are most common in small children, usually under the age of six.
They happen when a child stirs from non-REM sleep but can't reach full waking consciousness.
They are relatively rare and usually just happen for a few nights before stopping.
A child having a night terror will possibly sit up straight in bed, with his eyes open as if he were awake.
He may cry or shout out and look distressed. He will be confused and won't recognise his parents; he might not even realise that they are present.
This can last for anything from a few minutes up to half an hour. Afterwards he will go back into normal sleep.
Parents can find this very upsetting if they cannot comfort their child, but night terrors are nothing to seriously worry about. If they happen very frequently a GP may prescribe something to help the child sleep for a short time.
One way that night terrors are different from nightmares is that it is hard or impossible to wake the child up. Parents should just try to make sure that he is safe and comfortable rather than forcing him awake.
Another difference is that the child is unlikely to remember a night terror.
Night terrors tend to run in families and may occur if the child has had a fever or a stressful day. They can be more common if the child gets over tired, so make sure he goes to bed at a reasonable time and is getting sufficient sleep in general.
Night terrors usually happen in the first two or three hours after going to bed. You can sometimes prevent night terrors by timing when they usually start and waking the child up before this time.