Sleepiness is measured by a questionnaire (Karolinska Sleepiness Scale, KSS). Higher scores on KSS (range between 1 and 9) represent higher sleepiness.

Subjective sleep quality is measured by the Leeds Sleep Evaluation Questionnaire (LSEQ). Four different scales are evaluated: a) Getting to Sleep (range: 1-100, where lower values represent more difficulties to fall asleep), b) Quality of Sleep (range: 1-100, where lower values represent lower quality), c) Awake Following Sleep (range: 1-100, where lower values represent problems to wake up), and d) Behavior Following Wakening (range: 1-100, where lower values represent more sleepiness after awakening).

Working performance is measured by accuracy in an n-back task (%of correct responses in 2-back versus 0-back)

Vigilance performance is measured by reaction time patterns in a psychomotor vigilance task

Performance is quantified by assessing the course of the number of words recalled from a list of words (learned before treatment)

Cognitive performance is measured by the lure discrimination index and the recognition performance for repeat items.

Mental effort is measured by visual analogue scales targeting satisfaction, concentration, exhaustion and motivation. Scores range from 0-100. Higehr scores indicate higher levels on these scales.

Well-being is quantified as mean of three visual analogue scales targeting tension, mood and physical comfort. Scores range from 0-100. Higher scores indicate better well-being.

Using magnetic resonance imaging (MRI), we measure levels of cerebral blood flow in the brain during rest.

Questions regarding the participant's guess in which condition (caffeine vs placebo) they assume to have taken part (categorical: correct [1] vs incorrect [0]), how sure they are (scores range from 1-10, higher scores indicate higher certainty) and open question on reasons/indications for guess.

Craving for caffeine is measured using a visual analogue scale. Scores range from 0-100. Higher scores inidicate higher craving.