This study assessed the influence of smoking on diurnal variations in subjective activation and mood, with no restrictions placed on subjects' usual smoking habits after the first cigarette of the day. Forty-two students (21 smokers and 21 non-smokers) responded to questions on unipolar visual analogue scales every hour from 08:00 to 21:00 hours. At 08:00 h, the smokers were in a situation of nicotine deprivation. Each visual analogue scale was analyzed independently to determine whether they differ in sensitivity to time of day. We confirmed that subjective activation and mood improved after subjects smoked their first morning cigarette, with vigour, sleepiness and sadness being the most sensitive scales. Smokers' diurnal functions on negative scales of activation varied within a wider range than did non-smokers' and optimal moments were delayed 3 h as a result of worse morning ratings and greater post-lunch interference. The only mood scale sensitive to smoking was happiness, with smokers having a lower mean score than non-smokers, and a delay of 6 h in peak ratings. These results support reports of differences in the circadian rhythmicity of smokers and non-smokers, even when the former smoke at will. Smokers' presentation of suboptimal levels of activation and mood at critical diurnal periods is consistent with models of deprivation-reversal and acute nicotine depletion over the course of the day. Copyright 2000 John Wiley & Sons, Ltd.