A cardiac pacemaker device for monitoring a patient's heart rate variability comprising: means configured to deliver a cardiac pacing therapy including delivery of cardiac pacing pulses to trigger paced depolarizations; a sensing electrode (24, 26) configured to detect a cardiac depolarization; a sense amplifier (204, 152) coupled to the sensing electrode and configured to produce a signal indicative of a detected cardiac depolarization; an activity sensor configured to generate a signal indicative of activity; processing means configured to measure intervals between successive cardiac depolarizations, including intervals initiated by said paced depolarizations; a timing means configured to define successive shorter time periods during a longer monitoring period; processing means configured to use the signal indicative of activity for determining whether a patient is active; processing means configured to use data from the signal indicative of activity for determining rest periods that extend as long as the shorter time period; processing means configured to determine cardiac intervals occurring during each rest period; storing means configured to store the measured cardiac intervals; processing means configured to calculate a central tendency metric of the measured cardiac intervals during each rest period and to determine, in response to calculating the central tendency metric, a coefficient of variation of the cardiac intervals during the rest periods within the longer monitoring period to generate the heart rate variability; and means configured to employ the heart rate variability to modify the therapy delivered by the means for delivering a cardiac pacing therapy. means configured to deliver a cardiac pacing therapy including delivery of cardiac pacing pulses to trigger paced depolarizations; a sensing electrode (24, 26) configured to detect a cardiac depolarization; a sense amplifier (204, 152) coupled to the sensing electrode and configured to produce a signal indicative of a detected cardiac depolarization; an activity sensor configured to generate a signal indicative of activity; processing means configured to measure intervals between successive cardiac depolarizations, including intervals initiated by said paced depolarizations; a timing means configured to define successive shorter time periods during a longer monitoring period; processing means configured to use the signal indicative of activity for determining whether a patient is active; processing means configured to use data from the signal indicative of activity for determining rest periods that extend as long as the shorter time period; processing means configured to determine cardiac intervals occurring during each rest period; storing means configured to store the measured cardiac intervals; processing means configured to calculate a central tendency metric of the measured cardiac intervals during each rest period and to determine, in response to calculating the central tendency metric, a coefficient of variation of the cardiac intervals during the rest periods within the longer monitoring period to generate the heart rate variability; and means configured to employ the heart rate variability to modify the therapy delivered by the means for delivering a cardiac pacing therapy.