• Tidal rhythms (periods ~ 12.4 hours)• fiddler crab foraging

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”

Actogram showing free-running & entrained rhythms in cricket singing

LL schedule

LD schedule

The animal’s behavior free-runswithout a zeitgeber

With a zeitgeberthe animal’s internalclock is reset each day by exogenouscues (the light-darkcycle)

SCN Lesions in hamsters

Environmental cues reset the internal circadian clock daily

Synchronization of internalclock with external cues via entrainment pathways

The pacemaker sends out signals to coordinate physiology & behavior

Endogenous rhythms

The rhythms persist even when animals are kept under constant conditions

Infradian rhythms within the annual hibernation period: