Phase response curves to light in young and old hamsters.

The phase-shifting effects of 1-h light pulses on the circadian rhythm of locomotor activity were measured in young (less than 12 mo old) and old (greater than 16 mo old) hamsters. Phase response curves (PRCs) for both age groups showed an inactive region [approximately circadian time (CT) 0 through CT12], a delay region (CT12 through CT16), and an advance region (CT16 through CT24) as has been reported for young animals. Significant age group differences in the amplitude of phase shifts were measured, with older animals showing larger shifts limited to the region of the "break point" at CT16. The free-running period of the activity rhythm was measured before the first light pulse; age-related decreases of period length consistent with previous reports were measured. The findings indicate that the response of the circadian clock to the major environmental synchronizing agent, light, is different in old hamsters compared with young adults.