Caffeine, placebos and reaction time

Introduction: Caffeine is a stimulant, a drug that typically increases activity in the nervous system. Researchers tested whether a dose of caffeine makes people respond faster, whether simply believing you have had caffeine has the same effect, and whether people who use caffeine every day get less benefit from a dose than people who rarely use it.

Participants: Ninety adults aged 18 to 30 (48 women and 42 men) living in a mid-sized city in Canada responded to online advertisements and were paid $25. They described their race/ethnicity as 52% White, 21% East Asian, 14% South Asian, 8% Black and 5% another group. On a screening questionnaire, 45 participants reported consuming at least 300 mg of caffeine every day (about three cups of coffee); they formed the daily-user group. The other 45 reported consuming less than 50 mg per day; they formed the light-user group. People with heart conditions, high blood pressure or an anxiety disorder, and people who were pregnant, were not allowed to take part.

Method: A university research ethics board approved the study. Before agreeing to take part, participants read and signed a consent form explaining that they might swallow a capsule containing caffeine (200 mg, about the amount in two cups of coffee) or a capsule with no active ingredient, and that they could stop at any time. All participants were asked to have no caffeine after 8:00 p.m. the evening before their 9:00 a.m. session.

Within each user group, participants were randomly assigned to one of three conditions (15 people per condition in each user group): a caffeine capsule, a placebo capsule that looked and tasted identical but contained no caffeine, or no capsule. For the two capsule conditions, neither the participant nor the researcher running the session knew which capsule had been given.

Forty-five minutes after the capsule (or after arriving, for the no-capsule condition), participants completed a 10-minute task in which a light appeared on a screen at unpredictable moments 100 times. They pressed a button as soon as they saw the light. Alertness was measured as each participant's mean reaction time, in milliseconds (ms), across the 100 trials; lower times mean faster responding. At the end of the session, participants were told which condition they had been in and why a placebo was used.

Results and Discussion: Mean reaction times are shown in the table. For light users, the caffeine condition responded faster than the placebo condition, and this difference was statistically significant (p < .01). For daily users, the caffeine condition was also slightly faster than the placebo condition, but the difference was not statistically significant (p = .19). In neither user group was the difference between the placebo and no-capsule conditions statistically significant. The researchers concluded that people who use caffeine every day may get less benefit from the same dose.