Sleep to get the rest you need every night, but rest assured, your heart does not sleep. It slows down, a state called bradycardia, for part of the night, but may speed up, too. Your heart stays on alert, varying its rate for your body's changing needs in different sleep stages. Physicians at the Mayo Clinic define bradycardia as a heart rate under 50 beats per minute. Trained athletes and normal sleepers may safely drop that low.
Falling Asleep
When you lie down to sleep, your heart takes a break from pumping blood uphill from your feet, and from your heart to your head. Your body also uses less energy for balance and movement, slowing to meet lower energy demands. Your mind also sends a slow-down message to your heart. In 2001, Dr. Kurt Krauchi, reporting in "Neuropsychopharmacology," observed that heart rates in sleep-study subjects started to drop at the moment of a "lights out" signal. Overall, heart rate declines by about 8 percent from normal of about 72 beats per minute during sleep onset.
Dreaming
You dream for about one-fifth to one-fourth of a normal night's sleep, a stage called rapid-eye-movement, or REM sleep, even if you don't remember it when you wake up. Your heart is released from its usual neurological and physiological controls during this time, and its rate varies unpredictably over a wide range above and below normal. The rate may vary because of the emotional content in your dreams.
Non-REM Sleep
When you are not dreaming, during sleep stages 1 through 4, about 75 to 80 percent of normal sleep, your heart is under more direct nervous and metabolic control than in REM sleep. Your heart stays near its sleep-onset, 8 percent lower rate. But relaxation and neurological controls lower your body temperature during deepest sleep. Your heart rate may decrease by a few percent more. Sleep shifts through 90-minute cycles of stage 1 through 4 and REM all night. Your heart rate varies all night, and is also influenced by your physical conditioning and the kind of work you did all day.
Heart Disease
An injured heart does not respond in normal ways to sleep signals from your brain. Reporting in a 1995 edition of the journal "Circulation," a research team at the University of Oklahoma described heart rate changes they observed in sleeping patients after a heart attack. The normal slow-down signal transmitted through the vagus nerve became ineffective, allowing higher and more variable heart rates in sleep, unlike with normal subjects.
Sleep Disorders
If you use a continuous positive airway pressure, or CPAP, machine for obstructive sleep apnea, your heart rate is subject to other than normal influences. Some CPAP machines are prescribed with extra oxygen. This relieves your heart from speeding up in response to low oxygen levels. If your sleep apnea is only partially controlled, or you remove your mask during the night, your airway may re-obstruct as many as 200 times per hour, sending your heart rate to emergency high levels. With sleep apnea, your heart rate is highly variable.
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