Heart rate oscillates in synchrony with respiration. Several methods have been employed to assess this 'sinus arrhythmia', as an index of autonomic nervous system function. This paper proposes a new, easily computed measure, R, which is relatively resistant to the major nonrespiratory sources of variation, including premature beats, heart-rate differences among subjects, and slow trends in heart rate over time within subjects. The method can also be used more generally in any context where individuals are associated with event processes (for example, with seizures), when one requires assessment of the extent to which a point process is periodic with a particular known period.