Normal sleep is an uninterrupted and unconscious resting time your body and mind need for seven to eight hours out of every 24. Sleep slows and relaxes your heart and circulatory system because your metabolic needs are lower. So predicting your normal sleeping heart rate depends on your normal waking heart rate. And all your hours of sleep are not alike, either.
Dr. K. Krauchi, in a study reported in "Neuropsychopharmacology" (2001), detected an average drop from 64 to 52 beats per minute, about 8 percent, by the end of his subjects' gradual sleep onset. He also reported that the mere signaling of "lights out" to subjects started the drop. Researchers at the Abramson Center for Medical Physics at Tel Aviv University in Isreal, in 2006, discovered that during sleep onset your body transitions between two different physiological states. Nerves that slow your heart are activated during sleep onset and nerves that speed your heart are suppressed.
Subtracting 8 percent from your awake-resting heart rate approximates the drop that will begin at lights off and continue until you reach light, continuous sleep (stage 1).
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.
Twenty percent of the night, someone watching you sleep will see rapid-eye-movements (REM) under your sleeping eye lids--you're watching all the action fly by in your dreams. In REM sleep, your nervous system transitions to another physiological control state. Your heart rate and breathing are freed to vary widely and unpredictably and may even exceed waking norms. There is no "normal" heart rate in REM sleep.
Normal sleep is a series of approximately 90-minute cycles from stage 1 through stage 4, followed by REM, then back to stage 1. As a result, your heart rate changes frequently all night.
After a Heart Attack
After a heart attack--a myocardial infarction (MI)--a patient's heart responds differently in sleep than before. It may not slow from its waking rates. According to Dr. Emilio Vanoli, in the Cardiovascular Disease Section, at the University of Oklahoma, the nerves that slow your heart are not activated during sleep onset, and the nerves that can speed your heart remain active. As a result, loss of calming influences leave a patient's heart more vulnerable during sleep, possibly explaining the higher incidence of heart attacks and death during sleep.
Sleep apnea is a periodic upper airway closure (obstructive sleep apnea, OSA) or periodic absence of nerve impulses to breathe (central sleep apnea, CSA), which can occur from five to 200 times an hour, most commonly during REM sleep, but possible in other stages, too. This sometimes causes severe oxygen deprivation, circulatory changes and heart stress along with changes in heart rates. The actual rate depends on factors such as previous MI, cardiac medications, continuous positive airway pressure therapy (CPAP) for OSA, oxygen administration and many others. No "normal" sleeping heart rate can be predicted in sleep apnea patients.