Your heart rate slows down when you sleep, a natural physiological process that allows for rest and recovery. The average heart rate while sleeping typically decreases by about 10% to 30% compared to your daytime resting rate. This reduction results from the body shifting from alertness to deep relaxation and energy conservation. The cardiovascular system works less because the body’s overall metabolic demand drops substantially during the night.
The Body’s Mechanism for Slowing the Heart
The primary control for this nightly slowdown resides in the Autonomic Nervous System (ANS). The ANS is composed of two main branches that work in opposition: the sympathetic nervous system (“fight or flight”) and the parasympathetic nervous system (“rest and digest”). During the day, the sympathetic system is more dominant, keeping the heart rate elevated in anticipation of activity or stress.
As you fall asleep, there is a distinct shift in balance, with the parasympathetic nervous system becoming the dominant influence. This shift is associated with an increase in vagal tone, which acts like a brake on the heart via the vagus nerve. The parasympathetic activity signals the heart to slow its rhythm, allowing for lower blood pressure and reduced cardiac work.
This physiological change aligns with a substantial drop in the body’s overall energy expenditure and basal metabolic rate. Metabolic processes, such as breathing and temperature regulation, slow down, meaning less oxygen and blood need to be circulated. This reduced demand reinforces the need for a slower, more efficient heart rate controlled by the parasympathetic system.
Heart Rate Changes Across Sleep Stages
The heart rate does not remain uniformly slow throughout the night; instead, it fluctuates with the body’s cycling through different sleep stages. Sleep is generally divided into two main categories: Non-Rapid Eye Movement (NREM) sleep and Rapid Eye Movement (REM) sleep. Heart rate changes are most consistent and stable during the NREM stages.
During the deepest stages of NREM sleep, the heart rate typically reaches its lowest and most consistent point of the entire 24-hour cycle. The parasympathetic activity is at its peak during this deep sleep, resulting in the greatest reduction in heart rate. This stability allows the cardiovascular system to achieve its most profound state of rest and repair.
When the body transitions to REM sleep, the pattern reverses, becoming much more dynamic. REM sleep is characterized by high levels of brain activity similar to wakefulness and is associated with intense dreaming. The sympathetic nervous system becomes partially reactivated, causing the heart rate to increase and become highly variable. During REM, the heart rate can suddenly spike and fluctuate rapidly, sometimes reaching levels comparable to the waking resting rate.
Defining a Healthy Sleeping Heart Rate
Defining a healthy sleeping heart rate depends heavily on an individual’s fitness level and age. For most healthy adults, the sleeping heart rate generally falls between 40 and 60 beats per minute (bpm). However, for highly conditioned endurance athletes, a rate in the low 40s is considered normal because their hearts are exceptionally efficient.
The sleeping rate is an extension of the Resting Heart Rate (RHR), which is the rate when you are awake but calm, typically 60 to 100 bpm. Factors like age, body weight, and genetics influence where a person falls within the normal range. A healthy drop is generally around 20% to 30% below the waking RHR.
A consistently low rate below 40 bpm, known as pathological bradycardia, can be concerning if not explained by high fitness, and may indicate a heart conduction issue. Conversely, a sleeping heart rate that remains consistently above 100 bpm, or shows excessive and unexplained variability, can also signal an underlying issue. Conditions such as sleep apnea cause repeated interruptions in breathing, leading to sudden, sharp spikes in heart rate as the body attempts to restart breathing.