When I am having a nightmare, it's as if I am being possessed. I bear no resemblance to the person I am during the day. In fact, there have been times over the years when I hoped I was possessed because then I would have had a chance of getting rid of whoever was inside me.

I am 36 and have been a chronic sleepwalker for the past 20 years. The only way to describe what I am experiencing is to say that it's like a fear. I don't have an axe-wielding man chasing me or anything. It's simply pure fear without any remembered context. I am told by people who have seen me having a nightmare that I start thrashing about and then I move. And when I move, there is no stopping me. I just have to get out.

I have smashed through five windows in different places - a flat in Shepherd's Bush, a hostel in Australia, a window over the conservatory in my parents' house. Some years ago I was living in a mansion block in Maida Vale, London, with a couple. They were away and I was alone. I must have been having a terrible nightmare and smashed through my bedroom window. It is always the same thing that wakes me up: the pain of the glass shattering against my skin, or the noise of it, or the feeling of being outside. And when I wake up, I don't know what has happened, and may never know unless someone tells me.

If I am sleeping in a new place, I always check the strength of the windows and make sure I am somewhere safe. I never leave windows open and I always lock doors. I wouldn't want to sleep in a room too high up, but neither would I want to sleep on the ground floor. I live in a first-floor flat, which is perfect because there is a staircase, an alarm system and a locked front door between me and the outside world. Maybe it's because I'm a twin, but I prefer to have people around me.

Meeting someone new can be embarrassing. In the past when I have embarked on a relationship, if there was a possibility they might stay the night, I had to tell them what to do in the event of a nightmare. It's not exactly conducive to romance. No one has been put off me by seeing me sleepwalk, but I know that I have terrified people: sleepwalkers are frightening because you are never sure what they are going to do. I am in a state of fear and will defend myself: if you touch me, I could grab hold of you. My twin sister sleepwalks, too, and I can't bear to go anywhere near her when she is having a nightmare. It's the screaming I can't stand: it sounds horrific. Although I have my eyes open, I don't walk with my arms stretched out like they do in old movies. But I know I am scary.

I met Greg in Sydney - we worked for the same advertising company - and we have been together for about three years. When we started living together, my sleepwalking became so bad it was affecting our relationship. We were permanently tired, and I knew I had to do something. Back in London, I was referred to St Thomas' hospital and finally found the answers I had been looking for. There are four stages of sleep, and restorative sleep takes place in stages three and four. I have a chemical imbalance that means my switching mechanism between stages of sleep doesn't operate. If you look at my graph, you can see I was only dipping into stage three and never went into four, because I was waking myself up by sleepwalking.

For a long time I had been unwilling to become reliant on drugs, because no one had been able to explain what was happening to me. Now they have done so, I am on a medication for epilepsy (it stops seizures, but is also used for sleep disorders). I wake up in the morning feeling concreted into bed. After years of never having proper sleep, I feel rested. And for the first time I have started to dream.

Greg and I are getting married soon. I am hoping to have a baby and if I get pregnant, I won't be able to take the medication. That will mean I am back where I started, but I have spent most of my life dealing with nightmares - and there's a chance the hormonal changes during pregnancy will help to stop the sleepwalking. Because the condition is genetic, my child may sleepwalk, too. But at least I will know how to reassure him or her. At least I will understand the fear.