1. Tidal rhythms (periods ~ 12.4 hours) fiddler crab foraging

3. Free running vs Entrained In constant conditions (DD), the period is slightly less than 24 hours, so each day it starts earlier In a predictable light cycle (LD), wheel running becomes entrained to a 24 hour period Light is the zeitgeber = “time giver”

4. Actogram showing free-running & entrained rhythms in cricket singing LL schedule LD schedule The animal’s behavior free-runs without a zeitgeber With a zeitgeber the animal’s internal clock is reset each day by exogenous cues (the light-dark cycle)

5. SCN Lesions in hamsters

6. Environmental cues reset the internal circadian clock daily Synchronization of internal clock with external cues via entrainment pathways The pacemaker sends out signals to coordinate physiology & behavior

7. Endogenous rhythms The rhythms persist even when animals are kept under constant conditions

8. Infradian rhythms within the annual hibernation period: