Sleep spindles are 12-15 Hz electroencephalographic (EEG) oscillations characteristic of non-rapid eye movement (non-REM) sleep. Sleep EEG recordings (total 252 episodes; duration, 9 h 20 min each) distributed across the circadian cycle were analyzed in eight young and eight elderly human subjects. In the young subjects, sleep spindle amplitude, frequency, duration and incidence displayed significant circadian modulation, with the highest spindle incidence and amplitude and the lowest spindle frequency coincident with circadian phases associated with times of habitual sleep. In the elderly subjects, overall amplitude, duration, and incidence were reduced, and only spindle frequency exhibited a detectable circadian modulation. The results demonstrate that a circadian process modulates sleep spindle characteristics and that the strength of this circadian modulation is reduced with age.