A Welsh psychiatrist has claimed that some iPod users are experiencing what he calls "musical hallucinations".

Dr Victor Aziz said that the effect can occur when people spend many hours listening to the same songs. The hallucinations are characterised by a song " playing" constantly in the head, according to the psychiatrist, and the effect has caused sleeping problems for some of his patients.

"People find they can't sleep and can't think properly," he told the Evening Standard. "Having a song in your head every now and then is quite normal, but musical hallucinations can be quite distressing."

Dr Aziz first came across the phenomenon seven years ago when a heart bypass patient named Reginald King was referred to him.

King said that he began hearing pop songs and classical music and has done so every day since. Often one song runs into another rather like listening to tracks on a sound system. Dr Aziz called the experience "musical hallucinations ".

Dr Aziz belongs to a small circle of psychiatrists and neurologists who are investigating this condition. They suspect that the hallucinations experienced by King and others are a result of malfunctioning brain networks that normally allow us to perceive music.

He believes that this condition has existed for centuries but that the use of personal music systems has exacerbated the situation because they provide a stream of music repeated many times.

In the July issue of Psychopathology, Dr Aziz and his colleague Dr Nick Warner published an analysis of 30 cases of 'musical hallucination' over 15 years in South Wales. It is the largest case-series ever published for the phenomenon.

"We were trying to collect as much information about the [patients'] day-to-day lives as we could," Dr Aziz said.

"We were asking a lot of the questions that weren't answered in previous research. What do they hear, for example? Is it nearby or is it at a long distance?"

There is no standard procedure for treating musical hallucinations. Some doctors use antipsychotic drugs, and some use therapy to help patients understand what's going on in their brains.

"Sometimes simple things can be the cure," Dr Aziz said. "Turning on the radio may be more important than giving medication."

Dr Aziz suspects that musical hallucinations will become more common because of increased exposure to music from a variety of media.