A rabbit is active at dawn & dusk. It's normal activity is recorded in a lab for 6 days, then recorded in constant darkness for another 10 days. There s still a pattern to the rabbits behavior but it starts 20 mins later each day.

• Zeitgeber
• Entrained
• Biological Clock
• Endogenous
• Exogenous
• Circadian
• Circatidal
• Phase Shift 
• period of..
• Nocturnal
• Diurnal
• Crepuscular
• Biorythm

A rabbit is crepuscular. It's normal exogenous rhythm was recorded for 6 days with the zeitgeber present, which was entrained the rhythm. When the rabbit's activity was recorded in darkness the zeitgeber was removed & the activity pattern became free running which means the biorhythm is endogenous and the rabbit's biological clock is helping to run it.