MAMMALIAN ENDOGENOUS ACTIVITY RHYTHMS 795 



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Fig. 3. Twenty-five-day record of running activity of a female Peromys- 

 cus. Initial rhythm in constant darkness 22 hr 50 min. Twelve-hour light 

 period indicated by horizontal line under daily record. Coordinates as in 

 Fig. 1. 



A mouse with an endogenous rhythm longer than 24 hr is also able 

 to synchronize its activity to a 24-hr light cycle. In Fig. 4 the record 

 of such a mouse is given. Spans of 2 hr of light per day were presented 

 for nine consecutive days and are represented by the short horizontal 

 lines under the daily activity records. In this case the light fails to 

 cause a delay in the time of onset of the active period, but rather stops 

 the progressive shift of the rhythm, synchronizing it to 24 hr. Another 

 Peromyscus which had a period in constant darkness of 24 hr 10 min 



