858 PERIODIC FUNCTIONS IN MAMMALS 



subjects on the shifted schedule were up and about until 3:30 a.m. 

 and slept until noon in a room shielded from light and sound. A shift 

 of rhythm (by '^'90°) may be seen for this group as compared with 

 that on a regular schedule (to bed at 9:30 p.m.; up at 6 a.m.). More- 

 over, the shift of temperature rhythm seen early after the institution 

 of a new schedule persists 2 months thereafter (Fig. 28), and the t 

 of the mean temperature is comparable for the two groups, on the 

 regular and shifted schedules, respectively. Finally, Fig. 29 reveals 

 that the eosinophil rhythm of human beings also may be shifted by 

 changes in routine. 



It may be concluded that in man the lighting regimen and/ or the 

 daily routine can shift the timing of certain 24-hr rhythms (when this 

 problem is tested under rigorous conditions). Such a conclusion has 

 been statistically ascertained for the data in Fig. 29. Reported cases 

 in which a shift of rhythm in man was not accomplished by a change 

 in routine come to mind in this connection. Underlying the results of 

 such studies we may find, perhaps, the inadvertent maintenance of 

 some synchronizing information on a given "old" schedule and/or 

 the failure to allow for the necessary "shift time." It also is apparent 

 from the data shown herein that the shift can be accomplished by 

 manipulating the daily routine (including the lighting schedule), 

 other things being maintained the same as far as feasible. There are, 

 of course, many environmental factors, which in addition to the 

 lighting schedule, constitute our "routine" and thus govern the timing 

 of rhythms in man. Other factors, such as diet and oxygen ("power 

 supply"), in turn, constitute conditions that are necessary, yet in them- 

 selves ordinarily not sufficient for control of the timing of rhythm; e.g., 

 the eosinophil rhythm of human beings maintains its usual timing 

 under conditions of constant environmental temperature and four 

 equidistant-isocaloric meals per day (Halberg and Howard, 1958; 

 R. B. Howard, 1952). 



In viewing the problem of phase control by the environment, the 

 synchronizer, we cannot lose sight of those important cellular processes 

 and their humoral controls that are involved in the control of period. 

 From the available evidence it does not seem likely that the environ- 

 ment alone determines the "average" period of circadian rhythms, 

 even though it is undoubtedly the ultimate master of their phase. 



