Being without rhythm or regularity, as the pulse.

without regard for rhythm

not having a steady rhythm; "an arrhythmic heartbeat"

Having an abnormal heart beat (too fast [Tachycardia], too slow [Bradycardia] or irregular [Fibrillation]).

Describing animals which do not follow a particular daily activity pattern. They may be active at anytime during the day or night and often have short, frequent periods of activity between longer rests, like lions, for example.