An implantable cardiac stimulation device selects an arrhythmia therapy based upon the time of day. The device comprises an arrhythmia detector that detects an accelerated arrhythmia of a heart, and a data circuit that maintains a time record of cardiac cycle lengths developed during detected accelerated arrhythmias. A therapy circuit provides a selected therapy to the heart based upon the maintained time record of cardiac cycle lengths and the time of day.