Melatonin production decreases with advancing age, leading to insomnia and changes in circadian rhythmicity. Administration of melatonin in variable doses resulting in supraphysiological or physiological night-time blood levels of melatonin has been shown to improve sleep quality in the elderly. To study the effect of low doses of melatonin, which do not affect daytime blood melatonin concentrations, night-time milk containing 10-40 ng/l melatonin was used as a drink with meals. The effect of about 0.5 l night-time milk daily on sleep quality and circadian activity was studied in elderly institutionalized subjects in two long-term double-blind, placebo-controlled, crossover studies. Night-time milk was given for 8 weeks and normal day-time milk for 8 weeks with a 1-week washout period in between. In the first study, which was performed during spring with sleep quality evaluated subjectively by specially trained nurses, 70 demented patients showed only a seasonal effect on their sleep quality. In the second study performed around the winter solstice, 81 fairly healthy subjects living in rest-homes were divided into three groups, two for the crossover study as in the first investigation with a third group consuming only normal daytime milk as a control group to evaluate the effect of season. Caregivers graded the sleep quality and activity that was monitored separately for the morning before noon and for the evening after noon. In the second study, the effect of season was recognizable in the scores for sleep quality, which increased in all groups after the winter solstice. However, there were no changes in activity in the control group or in the group that consumed night-time milk during the first period of the crossover study, whereas both morning and evening activity increased significantly in the group that consumed night-time milk during the later period. Even ultra-low doses of melatonin may benefit the elderly by increasing their daytime activity.