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Data from five separate groups of volunteers show that diurnal variation in perceptions of physical and mental energy ('vigour') can be simply measured by the use of visual analogue scales. In all five groups, perceptions of vigour were significantly higher in the morning than at other times of day. In three groups where energy levels were measured six times daily the values achieved between 10.00 h and 12.00 h were significantly higher (P < 0.05) than those recorded on rising or retiring. Some evidence was also found for the existence of high energy levels in the evening (particularly among the undergraduates) and for an 'afternoon dip' in energy in about 20% of subjects. Higher levels of positive affect were also found in the morning and levels recorded between 10.00 h and 12.00 h were significantly greater than those on rising or retiring, (P < 0.01). However, no similar diurnal variation was observed for negative affect. The difference may be due to the fact that positive affect has a distinct 'biological' component contributing to perceptions of energy, whilst negative affect depends more on features in the individual's environment, which show no consistent diurnal pattern.