Too much coffee not only makes sleeping difficult, but can appear to wake the dead.

People who drink more than seven cups of instant coffee a day have an increased tendency to hallucinate — and may even think they sense ghosts, according to research.

High caffeine users were three times more likely to have heard a non-existent person’s voice than “low” users who consumed less than one cup of instant coffee or its equivalent, scientists found.

Besides coffee, caffeine can be obtained from sources such as tea, chocolate, “pep” pills and energy drinks.

The researchers from the University of Durham pointed out that hallucinations are not necessarily a sign of mental illness and around 3% of people regularly hear voices.

They studied 200 students who were asked about their typical intake of caffeine-containing products.

The students’ susceptibility to hallucinatory experiences and stress levels were also assessed.

Among the experiences reported were seeing things that were not there, hearing voices, and sensing the presence of dead people. Caffeine’s ability to exacerbate the effects of stress may explain the findings, the scientists believe.

When under stress, the body releases a hormone called cortisol, which is produced in greater quantities after consuming caffeine. The extra cortisol boost could be what helps a person hallucinate.

Psychologist Simon Jones, the PhD student who led the study, said: “This is a first step towards looking at the wider factors associated with hallucinations.

“Previous research has highlighted a number of important factors, such as childhood trauma, which may lead to clinically relevant hallucinations. Many such factors are thought to be linked to hallucinations, in part because of their impact on the body’s reaction to stress.

“Given the link between food and mood, and particularly between caffeine and the body’s response to stress, it seems sensible to examine what a nutritional perspective may add.”

Co-author Dr Charles Fernyhough, also from the University of Durham’s psychology department, pointed out that the study only showed an association between caffeine intake and hallucination proneness, not a causal link.

“One interpretation may be that those students who were more prone to hallucinations used caffeine to help cope with their experiences,” he said.

“More work is needed to establish whether caffeine consumption, and nutrition in general, has an impact on those kinds of hallucination that cause distress.”