PERIODICITY IN HUMAN BEINGS AND MICE 817 



by the eyes may eventually gain control of the timing of rhythm, and 

 this effect in turn may account for the tendency toward resynchroniza- 

 tion seen at the later time points after blinding. For the purpose of 

 analysis, these data thus are of twofold interest. 



First, they reveal that the time scale of physiologic 24-hr periodicity 

 need not always be synchronized with the environmental clock hour. 

 Of necessity we go by the local time in our work and, accordingly, we 

 exhibit the clock hour on the abscissas of our graphs. But in doing so, 

 we endeavor to study physiologic time relations rather than occur- 

 rences at a given clock hour. We can achieve the former goal reliably 

 only if we ascertain the degree to which the two time scales, the 

 physiologic and the environmental clock hour, are synchronized. 



Second, the data analyzed in Fig. 3 reveal the role played by sensory 

 inflow in the synchronization of the two time scales, that of the body 

 and that of the environment. It is from this point of view that we may 

 refer to the synchronizing effect as a result primarily of factors from 

 without. 



The terms "Zeitgeber," "clue," and "cue" correspond more or less 

 to "synchronizer" (Halberg, Visscher, and Bittner, 1954), the last 

 term being used herein partly following a line of thought expressed 

 earher by Kleitman (1949) (see also Kalmus, 1953). With reference 

 to "clues" it has been stated, however, that ". . . their importance Hes 

 in the maintenance of rhythm and in keeping it in phase with the en- 

 vironment" (Cloudsley-Thompson, 1956). 



The term "Zeitgeber," or "time-giver," in turn, has been qualified 

 so that its meaning is restricted to the synchronizing effect, and 

 "maintenance" is not included among the effects of a "time-giver" 

 (Aschoff, 1954, 1958). The definition'of "Zeitgeber" thus is identical 

 to that of synchronizer. 



It is important that in the absence of a given dominant synchronizer, 

 such as photic stimuli received by the eye, a given rhythm continues to 

 occur, even though it may do so with a different timing. Therefore, we 

 may postulate the existence of intrinsic mechanisms, which account 

 for the maintenance of rhythm and which normally interact with the 

 extrinsic synchronizer of rhythm. To use a crude and inadequate 

 analogy, we segregate those factors which "set the clock" along the 

 environmental time scale (synchronization, primarily, but not exclu- 



