Medical view: It's difficult - cases rarely black or white

The parasomnias include sleepwalking, of which there are several types, sleep paralysis (awakening with muscles paralysed), bed wetting and other disorders.

Sleepwalking disorders also include sexsomnia or sleep-sex, that is engaging in sexual behaviour while asleep.

With sexsomnia there are multiple disorders that can account for it and it is now recognised that, with the condition, all common forms of sexual activity can be exhibited.However, it's only in the last 14 years that we have been aware of sexsomnia.

In 1996, various works were published at the same time, including contributions by Colin Shapiro, who previously worked here in Scotland, based at the Royal Edinburgh Hospital. It can cause all kinds of marital disharmony - in fact that is usually how it comes to the attention of GPs.

Now there is some awareness that it is linked to a brain disorder so it is controllable - if you treat the disorder you treat the sexsomnia.

The thing about sexsomnia is that you are talking about an instinctive behaviour - sex.

There are sleep eating disorders, where people go to the fridge and make themselves some food - it's instinctive behaviour. Another common thing among sleepwalkers is urinating, which again is instinctive.

The problem, in terms of the sleepwalking defence, is that we will now get more people using it, and you are always worrying that perhaps people are lying to you. You want to be able to help the jury decide whether it is really sleep walking or not.

There have already been cases where people have concocted a story, or where people have used their sleepwalking as a way of masking illegal behaviour that they are involved in.

It can be very difficult. I've had very few cases where I can say definitely one way or the other.

It's difficult for the experts - most cases tend to be shades of grey, rather than black or white.

• Dr Chris Idzikowski works for the Edinburgh Sleep Centre.