794 PERIODIC FUNCTIONS IN MAMMALS 



this particular animal became active 50 min earlier each day or, in 

 other words, the period of its rhythm was 23 hr 10 min. 



The fact that the periods recorded simultaneously for a number of 

 Peromyscus range from values slightly less than 23 hr to slightly in 

 excess of 24 hr shows that no external cyclic clue is involved. Figure 

 2 illustrates this point, showing the times of onset of activity of a 

 female Peromyscus and her litter of six young born and raised in 

 constant darkness. Each young mouse was removed to a separate cage 

 on the day of the beginning of its appearance on this graph. The 

 divergence of the period lengths of the rhythms of the young mice and 

 their mother differs from the results of similar experiments obtained 

 by Aschoff (1955) for Mus, in which the young had approximately 

 the same period length as the mother. 



Although the 24-hr rhythm is basically endogenous, it is sensitive 

 to, and may be synchronized by, some environmental factors. Of these 

 factors light has the strongest effect. 



LIGHT EFFECTS 



The effect of constant light on nocturnal rodents is to cause a 

 daily delay in the time of onset of activity as long as the light persists. 

 A quantitative exponential relationship between the light intensity and 

 the magnitude of the daily delay of the midpoint of activity was first 

 shown by Johnson (1939) for Peromyscus. My own results confirm 

 this relation (Rawson, 1956). 



Presenting 12 hr of light during the period when the mouse is active 

 produces a delay equal to that produced by constant light. If the light 

 of the same intensity is presented only during the inactive period, l| 



there is no delaying effect on the rhythm. 



This differential response to light allows an animal with a rhythm 

 shorter than 24 hr to bring its endogenous activity rhythm into syn- 

 chrony with the 24-hr cycle of light and dark. Figure 3 shows the 

 synchronization of a rhythm of period shorter than 24 hr (actually 22 

 hr 50 min) to a 24-hr light cycle with 12 hr light per day. The double 

 horizontal line in the figure indicates the occurrence of the light. The 

 basic rhythm returned when the animal was again subjected to con- 

 stant darkness. 



