It's like the music group Bloodhound Gang says: "You and me, baby, ain't nothing but mammals." Other mammals also have rhythm-based body clocks that dictate when they rise in the morning and when they lie down for some shut-eye at night. In fact, bears depend on seasonal circadian rhythms to tell them when to hibernate for the winter and when to come out from their hiding places. Birds similarly fly south for the winter based on circadian (or circannual) rhythms that adapt to changes in temperature and the decreasing amount of sunlight per day [sources: Newitz, Quraishsi].
It isn't just mammals who keep time naturally either. Internal rhythms drive plants to open their leaves at dawn and close them at night. They also determine when flowers bloom seasonally [source: American Society of Plant Biologists].