Rhythmic vs homeostatic influences on mood, activation, and performance in young and old men.

Nine healthy old (80+ years) men were compared with nine healthy young (20-30 years) men in a protocol that required 36 hours of continuous wakeful bedrest. Body temperature rhythm measurement confirmed that the old had as robust an endogenous circadian (approximately 24 hours) rhythm generation mechanism as the young. However, in measures of affect, activation, visual search speed, verbal reasoning speed, manual dexterity, and vigilance hit rate, the old showed a linear decline over the 36 hours of the vigil, with little of the superimposed 24-hour rhythmicity that was apparent in the young. Thus, separate from the endogenous rhythm generation processes, there appeared to be some attenuation with advanced age, which led to the relative absence of rhythmic expression in these mood and performance variables. Such attenuation might contribute to some of the sleep and performance problems reported by elderly adults.