Various methods are used to quantify sedative drug effects, but it is unknown how these surrogate measures relate to clinically relevant sleepiness. This study assessed the sensitivity of different surrogates of sedation to clinically relevant sleepiness induced by sleep deprivation. Nine healthy volunteers completed a balanced three-way cross-over study with 1-week wash-out periods. Adaptive tracking, smooth-pursuit and saccadic eye movements, body sway, digit symbol substitution (DSST), visual analogue scales (VAS) and electroencephalograms (EEG) were evaluated on three occasions: (1) during the day after normal sleep, (2) during wakefulness at night; and (3) during the day after a night of sleep deprivation. VAS of alertness showed a gradual decline at night and a constant average reduction of 38 percent [95% Confidence intervals (CI), 28-47%] during the day after sleep deprivation. Average mood scores diminished by 14 percent (95%, CI 2-24%) during the day after sleep deprivation. Adaptive tracking, saccadic eye movements and body sway tended to deteriorate at night, but overall this was not statistically significant. After a night of sleep deprivation, adaptive tracking decreased by 21 percent (95% CI, 11-30%), saccadic eye movements decreased by 9-10 percent (95% CI, 5-13%/6-15%) and body sway increased by 37 percent (95% CI, 5-79%). In contrast, EEG beta2-amplitudes declined significantly at night by 18 percent (95% CI, 6-29%), without changes during the day after sleep deprivation. Smooth pursuit, DSST and other EEG-amplitudes remained unchanged. These results emphasize that reductions in adaptive tracking, saccadic peak velocity and body sway caused by sedative drugs really reflect sedation. They also provide a level of clinical significance for these surrogates of sedation. EEG parameters and smooth pursuit were unaffected by sleep deprivation, so drug-induced changes in these measures may not reflect sedation in a stricter sense. The motivation and alertness necessary for DSST may overcome mild sedation.