Sleep progresses from stage 1 through stage 4--progressively deeper stages of physical relaxation--which occupy about 80 percent of your sleep time. During this time your heart rate continues at its lower rate and may slow a few percent more as relaxation deepens, metabolism slows and your body temperature drops slightly. Differences in age (seniors have progressively less stage 3 and 4 sleep), general physical condition, your metabolic response to your previous day's work level and other factors prevent exact predictions of further heart rate decreases during stage 1 through stage 4.