844 PERIODIC FUNCTIONS IN MAMMALS 



and Dyke (1957, Pittendrigh (1954), and ourselves (Halberg, Vis- 

 scher, and Bittner, 1954). But in speaking of "synchronization," we 

 emphasize that herein this term is used to denote functions with equal 

 periods and it does not necessarily imply the occurrence of certain 

 events at identical times: synchronization in the mammal can be re- 

 garded as a state in which defined time relations (locked phase rela- 

 tions) with organismically determined phase differences (±) exist (or 

 can be brought about) among various functions (1) in a given organ, 

 (2) in different tissues, organs, and systems, (3) at various levels of 

 organization, and (4) between functions at one, several, or all of the 

 foregoing levels of integration on one hand and certain environmental 

 factors on the other. To what extent do the various aspects of organis- 

 mic synchronization (1 to 3 above) depend upon the environmental 

 synchronizer, and can they all be shifted by changing the schedule of 

 the dominant synchronizer? Will these shifts differ as a function of 

 differences in the complexity of phase control? For the system just 

 sketched we may predict that all 24-hr rhythms can eventually be 

 shifted ("pulled") by changing the dominant synchronizer and also 

 that the speed or ease of shifting may be different as a function, e.g., 

 of the "time constants" of the "phase comparators" involved. With 

 these considerations in mind, let us now turn to the effect upon various 

 24-hr rhythms of changes in the dominant synchronizer. We shall 

 refer particularly to effects of the lighting schedule in the mouse and 

 to those of more complex schedules in human beings. But at this point, 

 the reader interested in models of physiologic rhythms may be directed 

 to reviews (Pittendrigh, 1954; Pittendrigh and Bruce, 1957; Schmitt, 

 to be published). 



ENVIRONMENT: THE SYNCHRONIZER 



It is particularly with respect to gross motor activity, among other 

 functions of the body as a whole or of certain organ systems, that the 

 effect of light and darkness upon 24-hr rhythms in animals has been 

 most extensively investigated in the past. The broadest approach to 

 the problem was perhaps that carried out in Bykow's laboratories 

 (Bykow, 1953, 1954). O. P. Shcherbakova (1937, 1938, for dis- 

 cussion see Bykow, 1953) investigated species ranging from monkeys. 



