Heart Rate And Rem Sleep: A Complex Relationship

Sleep is a restorative process, so it makes sense that your heart rate declines during sleep. However, the extent to which it does so depends on the sleep stage you're in.

During light sleep, your heart rate gradually slows to your resting heart rate. In deep sleep, your heart rate decreases to its lowest levels and can drop 20-30% below your resting heart rate. However, in REM sleep, your heart rate can increase to a rate similar to when you are awake, especially if you are having an intense dream.

Heart rate during non-REM sleep

During non-REM sleep, the heart rate is at its lowest level. This is because the sleeper is in a state of deep sleep, and their blood pressure falls. The heart rate slows to about 20% to 30% below the individual's resting heart rate.

Non-REM sleep is associated with parasympathetic dominance, which is when the heart rate is lower and controlled by the vagus nerve. This is in contrast to REM sleep, which is marked by periods of higher activity and sympathetic dominance, where the heart rate is higher and controlled by the sympathetic nervous system.

The transition from non-REM sleep to REM sleep is associated with an increase in heart rate and blood pressure. This may be due to the withdrawal of vagal activity during REM sleep.

The heart rate changes during the different stages of sleep are likely due to the influence of sleep on the autonomic nervous system. Sleep has a strong interaction with the central nervous system and the autonomous nervous system.

Heart rate during REM sleep

Sleep is a restorative process, so it makes sense that your heart rate declines during sleep. However, the rate at which it declines depends on the type of sleep you're experiencing.

Sleep is divided into two primary stages: rapid-eye-movement (REM) sleep and non-rapid eye movement (NREM) sleep. NREM sleep can be further divided into three stages: light sleep, deeper sleep, and deep non-REM sleep, also known as slow-wave sleep.

During light sleep, your heart rate gradually slows to your resting heart rate. Deep sleep is when your heart rate is at its lowest, dropping 20% to 30% below your resting heart rate.

REM sleep, on the other hand, is when your heart rate may speed up to a rate similar to when you are awake. This is because it reflects the activity level occurring in your dreams. If you're having an intense or scary dream, your heart rate will increase as if you were experiencing those events in real life.

It's important to note that while your heart rate may increase during REM sleep, it still shouldn't exceed your normal daytime heart rate. If your heart rate doesn't decline during sleep or rises above your daytime resting rate, it could indicate a medical or psychological condition, such as anxiety or atrial fibrillation.

Additionally, a consistently low heart rate during sleep could be a sign of a sleep disorder like sleep apnea, which causes frequent pauses in breathing during sleep, disrupting your heart rate and compromising sleep quality.

Heart rate variability

Technical Considerations

Mathematical methods such as time- and frequency-domain analysis are used to study heart rate oscillations and, consequently, autonomic cardiac modulations. In this response, we will focus on the most frequent methods for exploring autonomic cardiac modulation.

Time-Domain Analysis

This method describes heart rate using a mean or standard deviation. The standard deviation of normal-to-normal intervals (SDNN) represents the variability over the entire recording period, obtaining the overall autonomic modulation regardless of sympathetic or parasympathetic arm. Other indices describe parasympathetic tone, calculated from differences between consecutive heartbeats, representing short-term variability. These measures include the root mean square of the successive differences (RMSSD), the number of interval differences of successive heartbeats greater than 50 ms (NN50), and the proportion of NN50 (pNN50, NN50 divided by the total number of heartbeats).

Frequency-Domain Analysis: Fourier Transforms

The Fourier transform decomposes a function according to its contained frequencies to build a spectral power spectrum for each frequency. To examine autonomic cardiac modulation in a heart rate Fourier spectrum, total spectral power (0-0.4 Hz) is considered (low-frequency [LF]: 0.04-0.15 Hz; high-frequency [HF]: 0.15-0.4 Hz). Total spectral power indicates overall HRV and allows for the assessment of overall autonomic cardiac modulation. HF power represents short-term heart variation. Studies have shown that injected atropine completely eliminated HF power, indicating that HF power is modulated by parasympathetic activity only, corresponding to peak respiratory rate (0.18-0.40 Hz). Pharmacological studies have shown that muscarinic cholinergic blocker (atropine) or beta-adrenergic blocker (β-blocker) lowered LF power, enhanced by dual blockade (atropine + β-blocker). Both parasympathetic and sympathetic cardiac activity would therefore be associated with heart power in the LF band. Normalized indices such as LF/HF ratio, LF% [LF/(LF + HF)*100], and HF% [HF/(LF + HF)*100] are used to examine this relationship.

Non-Linear Approach: Complexity of HRV

Alternatively, a non-linear approach has been proposed to study cardiac autonomic control. In recent years, there has been an emergent interest in non-linear dynamics that characterise autonomic cardiovascular control, leading to a growing literature. The study of the complexity of the different feedback loops impacting the cardiac function has led to novel indices capable of reflecting the complexity of the signal. Although several non-linear methods have been developed, we will briefly present entropy-derived measures, which have been recently applied for the assessment of autonomic cardiovascular complexity during sleep, such as approximate entropy, sample entropy, corrected conditional entropy, and Shannon entropy. The increase in the complexity of the cardiac signal, reflected by the increase in these non-linear indices, is usually associated with vagal modulation, and its decrease is usually interpreted as the result of an increased sympathetic drive and vagal withdrawal.

Time-Frequency Transforms: Transit Changes in HRV

Wavelet or Wigner-Ville transforms are time-frequency methods used to analyse heart rate by tracking signal frequency over time. By examining transit changes in LF and HF power and the LF/HF ratio, they describe sympathetic and parasympathetic activity over time, effectively characterising transit autonomic cardiac changes to short-time tasks. These methods allow for the study of transient changes in the autonomic nervous system for short periods, from seconds to minutes.

Heart rate and sleep disorders

Sleep is not just the absence of wakefulness but a regulated process with an important restorative function. Sleep disorders are conditions that affect the quality, timing, and amount of sleep. Getting the right amount of sleep is an essential component of good heart and brain health.

During sleep, the stimulation of your nervous system is reduced, and most of your body processes slow down. Within about five minutes of falling asleep, your heart rate gradually slows to its resting rate as you enter light sleep. Your body temperature drops, and your muscles relax. People typically spend about half the night in light sleep.

During the next phase, deep sleep, your blood pressure falls, and your heart rate slows to about 20-30% below your resting heart rate. However, during the REM sleep phase, your heart rate can vary quite a bit. If you have a scary dream or one that involves activity such as running, your heart rate rises as if you were awake.

Sleep disorders can cause sleep disruptions and make it difficult to get the sleep you need to stay healthy. They have been linked to a higher risk of cardiovascular disease. Poor sleep can cause major cardiovascular disease risk factors, including obesity, high blood pressure, and diabetes. Insufficient or irregular sleep can negatively influence diet, stress, and other lifestyle factors that can raise the risk of cardiovascular disease.

Excessive daytime sleepiness, a symptom of many sleep disorders, can also lead to a greater risk of cardiovascular disease. Sleep deprivation and sleep disorders have been linked to increased inflammation in the body, which can elevate your risk of cardiovascular issues.

People with sleep disorders such as sleep apnea, where the tissue in the back of the throat blocks the airway, are at a much greater risk of having high blood pressure. Obstructive sleep apnea causes your oxygen level to drop, and as a result, your heart beats faster, causing your blood pressure to rise. Over time, this can lead to an ongoing increase in blood pressure.

Sleep apnea can also cause or worsen congestive heart failure. The low oxygen levels and high blood pressure related to sleep apnea can cause the kind of damage that leads to congestive heart failure. The heart muscle is unable to handle the stress caused by the condition. If sleep apnea is treated, patients with congestive heart failure will see their heart function improve.

Heart rate and cardiovascular disease

Sleep is not just the absence of wakefulness but a regulated process with an important restorative function. During sleep, the stimulation of the nervous system is reduced, and most body processes slow down. Within about five minutes of falling asleep, the heart rate gradually slows to its resting rate as the body enters light sleep.

In normal subjects, the transition from wakefulness to non-rapid eye movement (non-REM) sleep is associated with a decrease in the percentage of power in the low-frequency band and a marked increase in the high-frequency band, indicating parasympathetic dominance, as expected in quiet sleep. During REM sleep, low-frequency power does not change, while high-frequency power decreases to values comparable to wakefulness.

In patients with a recent MI, the surge in high-frequency power during non-REM sleep is completely absent, indicating a loss of the ability to activate the cardiac vagus in a condition free of emotional or physical activity. This creates a condition of relative sympathetic dominance even during sleep, which is usually described as a period of vagal dominance and low risk for lethal events.

The knowledge that sleep represents a unique condition in which autonomic activity can be studied in the absence of factors such as physical activity and higher cortical functions is essential for understanding the variability of heart rate and its potential implications for cardiovascular disease.

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