Standard heart rate variability (HRV) techniques have been questioned in the sleep and autonomic fields as imprecise measures of sympathetic and parasympathetic activity. A new technique has emerged, known as phase-rectified signal averaging (PRSA). PRSA is used to quantify the quasi-periodic accelerations and decelerations in short-term heart rate, an effect that is normally masked by artifacts and noise. When applied to a signal of peak-to-peak (RR) time intervals, these quasiperiodicities can be used to estimate overall vagal activity, quantified as deceleration capacity (DC) and acceleration capacity (AC). We applied the PRSA analysis to a healthy cohort (ages 21-60 yr) enrolled in a clinical sleep trial, in which ECG data during wakefulness and sleep were available. We found that DC and AC were significantly attenuated with increasing age: a 0.27 ms/yr decrease in DC and a 0.29 ms/yr increase in AC (P<0.001). However, even in the older subjects, DC values were higher then previously found in people post-myocardial infarction. We also found a drop in percentage of normal-to-normal intervals where the current interval deviated>50 ms from the previous interval with age, with a decrease of 0.84%/yr. We did not find any differences between younger and older subjects with traditional HRV techniques, such as low-frequency or high-frequency power. Overall, the study provides normative PRSA data and suggests that PRSA is more sensitive than other HRV measurements. We propose that the decrease in DC and AC may be a sensitive marker for autonomic changes with aging. Further work will be required to determine whether the observed changes predict poorer cardiac health prognosis.