Night-to-night variability in the minutes of slow-wave sleep (SWS: combined minutes of stages 3 and 4 sleep) was examined for 50 healthy adults: 23 women (56-82 years old) and 27 men (54-74 years old). Visually rated SWS was more variable night to night than computer-evaluated SWS using power spectral techniques. The night-to-night correlation of the visually rated SWS was 0.61. The night-to-night correlation of the spectrally evaluated SWS was 0.91.