Subjective measure of the effect of caffeine

Subjective measure of the caffeine

Kilpelainen et al carried out research on the effect of caffeine on vigilance and cognitive performance during extended wakefulness in 20081. This section summarize the subjective method of the effect of caffeine on subjective sleepiness, mood and perceived task performance. Results (see illustration 1) shows that subjective sleepiness increased in day two and was significant. This suggest that caffeine intake by experimental group may not had any marked effect to keep their sleepiness low. Fatigue and vigor were negatively effected in the performance of both groups. However, despite the very poor descriptive statistics given by writer, fatigue and efficiency were highly significant and vigor was very highly significant. Although, confusion was not changed in placebo group, it was increased in caffeine groups.
Perceived task difficulty increased in both group. It was claimed that motivation was improved because there was no change in amount of effort or perceived task importance. Amount of effort put by the pilots could be also due to their military culture. Military pilot tends to have difference in amount of effort they put in their work.

Illustration 1. Subjective effectiveness of the effect of caffeine
Subjective measures by Variable measured Placebo Interpretation Caffeine Interpretation
Sleepiness subjective sleepiness (day two) increase significant increase significant
vigor decreased significant** decreased significant
fatigue increased significant* increased significant
feeling of efficiency increased significant* ns
confusion no change ns increase significanct
anger ns ns
tension ns ns
depression ns ns
memory disturbance ns ns
Perceived task performance
important of task no change ns no change ns
task difficulty increased significant* increased significant*
effort on task no change ns no change ns
motivation high high
sustained attained task decreased significant* stable
(* p <.01); (** p <.001); all other significant were at conventional level of p <.05)

Results (see illustration 2) shows average value of different caffeine withdrawal symptoms.

Overall placebo group has low median in most withdrawal effects. This do suggest that placebo group suffered slightly more in withdrawal effects. However, overall median of caffeine group were higher and all were significant at p <.05. This suggest that, caffeine group suffered less in the withdrawal symptoms that was measured ranging from moderate+ to moderate+++ with majority under moderate+++.The withdrawal symptoms like tiredness, urge to perform task, feeling of self-confidence, contentment, fuzziness, headache, grumpiness, desire to socialize, sweating, limb tremor, depression and fly like symptoms did not change significantly on either groups.

Illustration 2. Subjective caffeine withdrawal symptoms/ ill effects
Caffeine withdrawal symptoms Placebo median Interpretation Caffeine median Interpretation
Decreased energy 2.0 moderate 2.12 moderate +
Decreased wellbeing 1 * quiet a lot 2.24 moderate ++
Increased blurred vision 1.41 * quiet a lot to moderate 2.24 moderate ++
Increased nervousness 2.0 moderate 2.12 moderate ++
Increased sleepiness 2.37 moderate +++ 2.41 moderate +++
tiredness ns ns
contentment ns ns
fuzziness ns ns
headache ns ns
grumpiness ns ns
socialising ns ns
sweating ns ns
limb tremor ns ns
depression ns ns
self confidence ns ns
flu like symptoms ns ns
urge to perform task ns ns
Average value of 0 = extremely; 1= quiet a lot; 2= moderate, 2.12= moderate +, 2.24= moderate ++, 2.41= moderate +++; 3 = a little; 4= not at all
(* not significant); ( **all other significant were at conventional level of p <.05) (


Research approach

  • This research was an exploratory research to find out subjective measure of the effect of caffeine during sleep deprivation. This was sleepiness, mood, caffeine withdrawal symptoms, motivation and perceived performance.


  • A convenience sample of 15 Finnish military student pilots were randomly assigned. There were a total of three sessions, five student pilots participating in each session. The control group has eight pilots and the experimental group has seven pilots. Their average age was 23 to 24 years.
  • These participants were habitual consumers of caffeine in forms of tea or coffee. This was crucial because participants who were not habitual users of caffeine products could have experienced higher effects from the caffeine, even small amounts of caffeine product consumption.


  • Independent Variable (IV). IV is the caffeine. The experimental group received 200 mg of caffeine and the control group received the placebo.
  • Dependent variable(DV). DV is subjective sleepiness, mood, caffeine withdrawal symptoms, motivation and perceived task performance.


  • A placebo and 200 mg of caffeine were used to measure the vigilance and cognitive skills.
  • Sleep dairy were used to keep record of participant's sleeping records 4 days prior to experiment.
  • As part of subjective assessments, subjective sleepiness was evaluated by using the Standard Sleepiness Scale. Participants were asked to choose a scale rating ranging from 1 ( feeling active and vital; alert; wide awake to 7 (Almost in reverie; sleep onset soon; loosing the struggle to remain awake).
  • Mood was measured using the Profile of Mood States Questionnaire using 38 adjectives are used describe subjective feelings of each of the questions rated from 0 (not at all) to 4 (extremely). Additionally, caffeine withdrawal symptoms/ill effects questionnaire were used using rating same scale of 0 (not at all) to 4 (extremely).
  • To measure motivation and perceived performance, a 10 cm visual analogue scale (VAS) were used. Four questions was used and each anchored with phrase of 'not at all' and 'very much'.


  • All 15 pilots were randomly selected in double-blind fashion so they received either the placebo (n=8) or caffeine in a capsule (n=7). All participants received the placebo before the first simulator. This was done in order to determine a baseline level and also to measure if there was any differences between the groups.

Data analysis

  • SPSS 14.0 was used to carry out data analysis. Non-parametric namely Friedman's two- way ANOVA and Wilcoxon signed- rank test were applied.

Generalization potential

  • Limited generalization. The generalization can only be applied to a group of 24 year old military student pilots who are habitual caffeine drinkers. Apart from these results there appears to be little relevance to other populations and age groups.
1. KILPELAINEN Airi A, Kerttu H HUTTUNEN, Jouni J LOHI & Heikki LYYTINEN (2008). Effect of caffeine on vigilance and cognitive performance during extended wakefulness. International Journal of Aviation Psychology, 2010, volume 20, number 2, pages 144-159.

Want to know more?

Objective measures of the effect of the caffeine. :The article provides objective measures of the effect of caffeine.

Caffeine in aviation : This pages briefly tells us role of caffeine in human body, effect of high dose and performances of pilot and ATC.

Cognitive performance : This pages tell us about improving cognitive performance.

Perception of energy drinks (caffeine) among student pilots. :The article provides perceived physical, psychological and practical effects of energy drinks amongst student pilots.

Original article : Full original article of this research can be obtained from here.

Contributors to this page

Authors / Editors

Abilash Thapa (2013), Massey University, New Zealand (Thapa AbilashThapa Abilash)

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