Too much coffee not just makes napping difficult but also dramatically increases the risk of hallucinating, according to a new study.
The research has found that people with a higher caffeine intake, from sources such as coffee, tea and caffeinated energy drinks, are more likely to report hallucinatory experiences such as hearing voices and seeing things that are not there.
'High caffeine users' can be defined as those individuals who consume more than the equivalent of seven cups of instant coffee a day.
Such drinkers were three times more likely to have heard a person's voice when there was no one there compared with 'low caffeine users' who consumed less than the equivalent of one cup of instant coffee a day, the study found.
In the study, funded by the Economic and Social Research Council and the Medical Research Council, 200 students were asked about their typical intake of caffeine containing products, such as coffee, tea and energy drinks as well as chocolate bars and caffeine tablets.
Their proneness to hallucinatory experiences, and their stress levels, were also assessed. Seeing things that were not there, hearing voices, and sensing the presence of dead people were amongst the experiences reported by some of the participants.
The researchers, whose paper is published in the academic journal Personality and Individual Differences, say their finding could be down to the fact that caffeine has been found to exacerbate the physiological effects of stress.
When under stress, the body releases a stress hormone called cortisol. More of this stress hormone is released in response to stress when people have recently had caffeine. It is this extra boost of cortisol which may link caffeine intake with an increased tendency to hallucinate, say the scientists.
Lead author, Simon Jones, a PhD student at Durham University's Psychology Department, 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."
