Sleeping heart rate should generally be somewhat lower than normal resting heart rate while awake, because the body typically relaxes very deeply during sleep. As a person begins to fall asleep, the heart rate begins to slow, and studies suggest that this process can begin as soon as a person knows he is preparing for sleep. As the body relaxes into a deep sleep state, core body temperature can decrease and metabolism usually slows, in addition to heart rate. Physical fitness level, age, and recent stress levels can all influence sleeping heart rate. Most experts believe, however, that normal heart rate during sleep should be eight to ten percent lower than normal resting heart rate while awake. A sleeping heart rate that is not at least eight percent lower than normal resting heart rate while awake could be a danger sign.
There are at least five stages of sleep, and sleeping heart rate can vary throughout each of the sleep stages. The first four stages of sleep, generally categorized as sleep stages one through four, occur as the body relaxes more and more deeply. This process of physiological relaxation accounts for about 80 percent of most peoples' sleep time. Heart rate usually begins to slow as stage one sleep is entered, and slows further as the body relaxes further.
Heart rate can often vary widely during REM sleep, the fifth stage of sleep during which dreaming usually occurs. Rapid eye movement (REM), sleep, is so named because the sleeping person's eye movements are usually visible to any observers. Physiological states can vary widely during REM sleep, possibly depending on the sleeper's emotional reactions to his dreams. Heart rate can increase considerably during REM sleep, and may even exceed normal resting heart rate while awake. Respiratory rates can also increase, and other physiological functions, such as perspiration, can occur.
Some evidence suggests that heart rate during sleep can be a good indication of possible mortality over the next seven years of the sleeper's life. An Israeli study suggests that people who do not experience at least an eight to ten percent reduction in heart rate while asleep may be as much as two-and-a-half times more likely to die within the next seven years of their lives. The study also seemed to suggest that people with chronic health conditions, such as diabetes, hypertension, and obesity, are probably most likely to experience the least reduction in heart rate while asleep.