Oscillation

Most of the organism's reactions show a tendency toward oscillation; that is, there is a somewhat cyclic ebb and flow between periods of hyperactivity and hypoactivity. We think at once of heart action, with its contraction period or systole rhythmically alternating with its relaxation period or diastole throughout life. Respiration, intestinal contraction, and other automatic activities exhibit the same phenomenon. But oscillation is not limited to the reflex level; such activities as color naming, verbal substitution, and blindfold maze-tracing show more or less periodic breaks. The close correspondence in reflexes between periods of hypoactivity and the refractory phase of the tissues involved has led many investigators to relate all types of oscillation with decrement and fatigue. Oscillation is said to be a self-imposed barrier to fatigue and to be determined by some intrinsic organic condition or "periodicity factor." This view has useful aspects, but it neglects the certain influence of extrinsic conditions. Variations in the rhythm of heart action occur in response to external stimuli. Reactions of the gastrointestinal tract can be "trained" for particular types of oscillation. If there is an intrinsic periodicity factor operating to determine the course of more complex reactions, it is most effectively masked. Performance does vary during the hour, the day, the season, and the year; but the key to an understanding of this variation does not lie in some mysterious intrinsic force as inevitable and as universal as the tide. Variations in behavior which correspond with time are resultants of a complex polygon of forces, including temperature, food ingestion, sleep, practice, distraction, effort and incentive, and exercise and fatigue. The way in which these factors are arranged in relation to the periods of work surveyed determines the form of oscillation which is exhibited.

Diurnal Oscillation

Recording the hourly level of output in different performances constitutes the most common approach to the problem of diurnal oscillation. The uncritical character of this work and the lack of general agreement in results are well known. In fact, somewhere in the literature one can find support for almost any kind of daily variation. Investigators are divided in opinion as to whether this indicates the absence of strictly intrinsic factors. Those who have used human subjects generally stress the importance of extrinsically conditioned factors, such as eating, incentive, and sleep. Those who have recorded the diurnal variations of lower organisms are inclined to feel that there is a basic internal factor which determines the character of the oscillation.

Some individuals do their best work in the morning, some in the afternoon, and others at night; and unless there are constitutional differences, the variations must be due mainly to a different organization of externally conditioned factors. If there is an internal physiological factor at work in producing diurnal oscillation in man, it is decidedly masked. Unfactored investigations which take no account of individual differences in habits of eating and sleeping or of the relation of these factors to variations in energy expenditure are in no wise crucial.

Diurnal energy expenditure can be measured most directly by the metabolic rate, but several related physiological processes have also been used as indicators. These include pulse rate, blood pressure, respiration, muscular tonus, and body temperature. Most investigators report that energy expenditure rises progressively throughout the day, but there are significant exceptions. The exceptions suggest that, like performance scores, energy expenditure varies with extrinsic conditions.