We are concerned here with the diurnal variations in human * output' and * input \ These measures are related intimately to the ' efficiency ' of performance, though just how we cannot yet say. The studies covered by this review have been grouped as follows: (I) those which deal with variations in work output alone; (II) those which record only some physiological measure of organic energy expenditure (or input) ; (III) those which correlate output records with concurrent physiological changes at different hours; and (IV) those which deal with factors affecting the diurnal curve. It seems advisable to include in these groups only the fluctuations occurring during the day. Johnson and Swan 1 have previously surveyed the literature on nocturnal variations.
(I) STUDIES IN WORK OUTPUTThis method constitutes the most obvious approach to the problem of diurnal variations. Education and industry early saw important implications in such research, and hardly a single human activity has escaped the attempt to plot its daily course. Considering the vast amount of work done, however, the results appear to the reviewers as rather inconsequential. Investigators differ not only as to the hour of maximum output, but also as to whether the morning or afternoon period is superior. Some results indicate a gradual increase in output during the morning and a gradual decrease during the afternoon. Others show a morning drop and an afternoon rise. In still others there appears to be little noticeable change. Much of 1