The search for cycles in mental performance from Lombarb to Kleitman

Summarizing this large volume of research, of which only part has been presented here, one cannot but admit that questions about the daily course of mental fatigue and its objective measurement, differences between 'morning' and 'evening' types, the phase relationship between physiological and behavioral rhythms, the relevance of task specific variables to rhythms in performance, are yet to be answered. It is the growing awareness of the rhythmic nature of biologic phenomena and the intensive investigation of sleep as an independent variable that influences waking behavior, that promise that the present wave of interest in rhythmic performance would not die out before proper answers are given.