Stranger than fiction

Ghostly faces and visions of 'little people': The eye disorder that leaves thousands of Britons fearing they've lost their senses

'The unpleasant feeling was of not being in control,' she says. 'Once they are identified, they are not a problem.'

Hallucinations can last from only a few seconds to several hours. In a minority of unlucky cases, they are continuous throughout the day. Patients usually have several daily before they taper off to once a week, then once a month.

For 60 per cent of patients, they will stop entirely after 18 months. There has not yet been a long-term study, but some patients report having them for at least three years.

Part of Dr ffytche's research involves looking into ways patients can stop the hallucinations. 'There won't be a single recipe for everyone,' he says. 'But hallucinations tend to occur when you are in a state of drowsy wakefulness, so you want to rouse yourself.'