Day sleep (after night work) and night sleep (after day work) were studied in two groups of locomotive engineers aged 25-35 and 50-60 a, respectively. All recordings were made in the homes of the subjects. For both groups day sleep was reduced by approximately 3.3 h, mainly affecting rapid eye movement sleep and stage 2 sleep. Diuresis and the excretion of noradrenaline were increased during day sleep. The ratings of sleepiness were higher after night work than after day work. Several indices of disturbed daytime sleep correlated significantly with catecholamine excretion. The age groups differed mainly in that the older subjects had relatively more stage shifts, awakenings, stage 1 sleep, a higher diuresis, and a higher noradrenaline excretion during day sleep. It was concluded that night work is detrimental to sleep and that negative effects are exacerbated by increasing age.