Response of circadian locomotor activity and the proestrous luteinizing hormone surge to phase shifts of the light-dark cycle in the hamster.

Female hamsters with regular 4 day estrous cycles were exposed to either a 3 hour phase advance or delay of the 14:10 light-dark (LD) cycle on the first, second or third day before proestrus. Blood samples were taken on proestrus to characterize the LH surge, and locomotor activity onset was recorded. Both the LH surge and activity onset phase delayed more quickly than they advanced, which can be explained by the free-running period of the hamster (longer than 24 hours). Higher estradiol levels were correlated with more rapid advances of activity onset.