The observation that your heart rate during sleep sometimes appears higher than your heart rate when you are awake and relaxed is a common, yet often misinterpreted, finding from personal health trackers. While the heart generally slows down significantly overnight, the measured “Sleeping Heart Rate” (SHR) is an average that incorporates normal, temporary spikes. These spikes are caused by natural sleep cycles, undetected medical conditions, or inaccurate data collection. Understanding the difference between the two measurements and the biological events that occur throughout the night helps explain this perceived paradox.
Defining Awake Resting Heart Rate and Sleep Heart Rate
The two measurements, Awake Resting Heart Rate (RHR) and Sleeping Heart Rate (SHR), represent distinct physiological states measured under different conditions. RHR is defined as the lowest stable heart rate achieved while a person is awake, calm, and stationary, typically measured immediately after waking up. It reflects the heart’s work rate under minimal physical or psychological strain.
SHR, by contrast, is usually reported as the average heart rate recorded throughout the entire night. This nightly average inherently includes periods of both deep slowing and temporary acceleration. During the most restful phases of sleep, particularly deep non-REM sleep, the heart rate can drop significantly below the RHR, sometimes reaching rates as low as 40 to 60 beats per minute (bpm). The SHR is an average that must account for all fluctuations across the sleep stages, unlike the RHR which is the lowest baseline rate achieved while awake.
Physiological Events That Raise Heart Rate During Sleep
The natural cycling through the stages of sleep is a primary biological reason for heart rate fluctuations. As the body enters non-REM sleep, the parasympathetic nervous system dominates, leading to a steady drop in heart rate and blood pressure. This restorative phase allows the heart to achieve its lowest and most relaxed state.
However, the transition into Rapid Eye Movement (REM) sleep, the stage associated with vivid dreaming, causes a shift in autonomic activity. During REM, the sympathetic nervous system—the body’s “fight or flight” regulator—becomes highly active, sometimes exceeding wakefulness levels. This sympathetic activation causes the heart rate and blood pressure to rise sharply, temporarily elevating the overall nightly average.
Certain medical conditions also cause frequent, intense heart rate spikes that elevate the average SHR. Obstructive sleep apnea (OSA) is a disorder where the airway collapses, causing a pause in breathing and a drop in blood oxygen levels (hypoxia). This lack of oxygen triggers a stress response from the sympathetic nervous system, causing an abrupt surge of adrenaline and a spike in heart rate to restore breathing. These repeated high-rate events, sometimes occurring dozens of times an hour, significantly raise the average heart rate for the entire sleep period.
Technical Factors and Wearable Device Inaccuracy
A higher measured Sleeping Heart Rate can often be an issue with how consumer wearable devices collect and process data. Most modern trackers use photoplethysmography (PPG), an optical technique that measures changes in blood volume under the skin. This method is highly susceptible to “motion artifacts,” which are errors caused by movement.
When a person tosses and turns or shifts position in bed, the slight movement of the wrist-worn sensor confuses the PPG light signal. This motion is often misinterpreted by the device’s algorithm as an actual pulse, leading to artificially inflated heart rate readings. The device’s averaging algorithm may heavily weigh these false high readings, resulting in a reported average SHR that is higher than the true RHR.
Inaccurate readings also stem from poor sensor contact or fit, which compromises the quality of the optical signal. Devices are generally more accurate when the wearer is completely still, such as when RHR is traditionally measured. However, continuous sleep tracking means the sensor must contend with hours of subtle, shifting contact. This increases the likelihood of data corruption and erroneous spikes in the recorded heart rate data.
Lifestyle and Environmental Influences on Overnight Heart Rate
External factors affecting the body’s metabolic state before or during the night can prevent the heart rate from dropping fully. Consuming alcohol close to bedtime is one cause, as the body works to metabolize the substance. Alcohol consumption increases the production of stress hormones like cortisol and adrenaline, which activates the sympathetic nervous system and can cause a measurable increase in nocturnal heart rate.
Eating a large or heavy meal late in the evening also forces the digestive system to remain active. Digestion requires significant blood flow and metabolic effort, which prevents the heart rate from achieving the deep reduction expected during rest. This digestive demand keeps the body’s internal systems active, elevating the overall average heart rate for the first few hours of sleep.
The ambient temperature of the sleep environment is another factor that can tax the cardiovascular system. If the room is too hot or too cold, the body must expend energy to thermoregulate and maintain a stable core temperature. This increased effort keeps the body’s metabolism and heart rate slightly elevated throughout the night, preventing the heart from reaching its lowest potential rate.