The effect of caffeine on reaction time

Dan Finnin, Josh Arndt, and Damaso Ahumada

CU Boulder, Fall 2008


We tested the influence of caffeine upon reaction time. Based on observations of increased alertness, we figured that reaction time would also increase as well. We hypothesized that caffeine will increase reaction time because of the physiological effects on the body.

We began by testing the reaction time of eight subjects with a reaction timer. We tested how fast a subject could react to either a color or sound in the test by pressing a button corresponding to the color or sound. We repeated the test ten times per subject. Once the subjects took the reaction test we gave the eight test subjects 160mg of caffeine in the form of a Monster energy drink.  We allowed 30 minutes to pass before repeating the test to allow the caffeine to take effect on the subjects. After dropping the fastest and slowest times of each subject, we compared the reaction times before and after consuming the caffeine.

Our results indicated a slight increase in reaction time after drinking the caffeine. The mean values were 0.506s prior to the drink and 0.491s after the drink. There is not a significant difference (P-value of 0.399).

Our results are inconsistent with our hypothesis. While there was improvement in some individuals, others had a decreased reaction time after application of the treatment. One reason for this is that we did not consider dependency on caffeine and levels of caffeine already present in the subjects. Also, that different combination of lights and sounds might cause different reaction times. Decreasing the number of options to press on the reaction timer to only one or two might eliminate this variable. Additionally, fatigue may play a factor in the results particularly from the first test to the second. Fatigued individuals might display an increase in reaction time whereas those that are alert will not see an effect.

A similar study to ours performed by Anderson & Horne (2008) did not use caffeine, but made the subjects think they had taken some in order to show significant improvement in reaction time for the treatment group versus the control group. Another similar experiment performed by Haskell et al. (2007) showed the effect of other additives to the energy drink may have had an impact on the results. They studied the effects of l-theanine, an amino acid found in tea, by itself, combined with caffeine, with just caffeine, and without either, on different cognitive tests. They found that there was improvement in some areas when l-theanine is added and detriment in others. In addition to caffeine, a Monster contains other ingredients that may have had an impact such as sucrose, taurine, or guarana. As a result of our experiment, we feel that our hypothesis and experiment need to be altered in order to better represent the data we are trying to prove. If we had standardized the amount of sleep, dependency on caffeine, the reaction test, and different additives, then we may have seen different results.


Anderson, C and Horne, J. 2008. Place response to caffeine improves reaction time performance in sleepy people. Human Psychopharmacol Clinical Experiments 2008, 23, 333-336.

Haskell, C, Kennedy, D, Milne, A, Wesnes, K and Scholey, A. 2007. The effects of L-theanine, caffeine and their combination on cognition and mood. Biological Psychology, 77, 113-122.