Professor Simon Crowe, from the School of Psychological Sciences, said:
"High caffeine levels in association with high levels of stressful life events interacted to produce higher levels of 'hallucination' in non-clinical participants, indication that further caution needs to be exercised with the use of this overtly 'safe' drug."
The 92 individuals were subjected to either a low or high stress condition, and a low or high caffeine condition. They had to listen to white noise and report whenever they heard the song "White Christmas", by Bing Crosby during the white noise. White noise is a background sound that contains every frequency within the range of human hearing - humans hear it as a constant fuzzy sound.
The White Christmas song was never played.
Those subjected to either high stress or high caffeine levels were more likely to self-report hearing the song, the researchers found.
Professor Crowe said:
"There is a link between high levels of stress and psychosis, and caffeine was found to correlate with hallucination proneness. The combination of caffeine and stress affect the likelihood of an individual experiencing a psychosis-like symptom."
Crowe added that their study also helped explain how stress can facilitate schizophrenia signs and symptoms in non-clinical samples. Caffeine has only recently been found to raise the risk of hallucinations.
Professor Crowe added:
"The results also support both the diathesis-stress model and the continuum theory of schizophrenia in that stress plays a role in the symptoms of schizophrenia and that everyone, to some degree, can experience these symptoms. This was demonstrated by a significant effect of stress on the occurrence of hallucinatory experiences, or hearing the song.
It is apparent that the health risks of excessive caffeine use must be addressed and caution should be raised with regards to the exacerbating use of this stimulant."
Coffee's stimulant effect is due to its caffeine content. How much caffeine there is in a cup of coffee depends on various factors, including the type of bean used and how it is brewed. The following measurements of caffeine in a cup of coffee are cited from an article by Bunker and McWilliams, from the Journal of the American Dietetic Association (74:28-32, 1979):
- 1 cup, brewed (7 oz, 207 ml). 80 to 135 mg of caffeine.
- 1 cup, drip (7 oz, 207 ml). 115 to 175 mg of caffeine
- 1 cup, espresso (1.5-2 oz, 45-60 ml). 100 mg of caffeine
Caffeine is also a diuretic (promotes the formation of urine by the kidney) when consumed in sufficient quantities. However, regular consumers eventually develop a tolerance to the diuretic effect.
Caffeine's chemical formula is C8H10N4O2.
"The effect of caffeine and stress on auditory hallucinations in a non-clinical sample"
S.F. Crowe, J. Barot, S. Caldow, J. D'Aspromonte, J. Dell'Orso, A. Di Clemente, K. Hanson, M. Kellett, S. Makhlota, B. McIvor, L. McKenzie, R. Norman, A. Thiru, M. Twyerould, S. Sapega
Personality and Individual Differences, Volume 50, Issue 5, April 2011, Pages 626-630