I was jet lagged and exhausted. After no more than a two-hour doze on the flight to New York, I'd stepped off the plane and ambled into a long day's work. It was the early hours of the next day before I made it back to my hotel room. I fell instantly into a deep sleep and had a remarkably lucid dream in which I was holding my baby son, wrapped in a colourful silk scarf. I could smell his skin.

I was wide awake by 6am (11, according to my body clock) and the room was filling with a grey light. The first voice, a few inches to my right, put a question: “Is that it?” The response came from the left: “Probably.”

Voice-hearing is usually read as a sign of serious mental illness, schizophrenia in particular. I have some experience of working with schizophrenia sufferers and know the signs and symptoms. So as the fear subsided, I reassured myself that I was not going mad. The voices resembled a typical schizophrenic “auditory hallucination” in some ways - they sounded real and “out there” rather than inside my head - but in other respects they did not. There was a simple question and response. Four bland words. My voices did not address me directly, and if they were my own thoughts I was hearing out loud then I have no idea what I was thinking.

Auditory hallucinations are not restricted to psychosis. They sometimes occur in severe depression and among survivors of childhood abuse. Then there are neurological causes, such as epilepsy. Perhaps voice-hearing should even be considered a part of normal experience.

Many of us enter strange states on the fringes of sleep from time to time. My New York experience would qualify as a “hypnopompic” hallucination - a false perception on awakening - and so can safely be categorised as “normal”. I knew that, but was still petrified. Why?

It's to do with the deep strangeness of the feeling that there were other articulate beings cohabiting my head. I remember once discussing voices with a sufferer of schizophrenia whose theory was that some of his brain circuits had disengaged from the rest to develop separate streams of thought. “I'm neuronally possessed,” he said. Perhaps I'd glimpsed what he meant.

Scientific surveys have revealed that 1 person in 50 admits hearing voices from time to time. Bereaved people are especially prone. A friend who recently lost his wife to cancer told me that she often spoke to him. He has no belief in the supernatural - the voice was testament to a relationship so long and loving that one mind had entwined with another.

Just the other night I was awoken by another voice. It spoke rapidly, and was telling me how to build a universe. Now there's a delusion of grandeur. Then, with relief, I realised I'd fallen asleep listening to my iPod, which had just shuffled to Chapter 1 of Bill Bryson's A Short History of Nearly Everything. I wonder if the others were listening.

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