Circadian rhythms are governed by a biological clock, and are known to occur in a variety of physiological processes. We report results on the circadian rhythm of heart rate observed using a wrist-worn wearable device (Fitbit), consisting of over 17,000 individuals over the course of 30 days. We obtain an underlying heart rate circadian rhythm from the time series heart rate by modeling the circadian rhythm as a sum over the first two Fourier harmonics. The first Fourier harmonic accounts for the approximate 24-hour rhythmicity of the body clock, while the second harmonic accounts for non-sinusoidal perturbations. From the diurnal modulation of heart rate, we obtain the following circadian parameters: (i) amplitude of modulation, (ii) bathyphase, and (iii) acrophase. We also consider the circadian rhythm of activity and show that in most individuals, the circadian rhythm of heart rate lags the circadian rhythm of activity. The widespread availability of smartwatches and trackers may enable individuals who are interested in observing their circadian rhythms of numerous physiological parameters, and to measure longitudinal changes in circadian parameters in response to various changes in health-related variables such as diet, sleep, exercise, or illness.