Definition and Overview
Sleep and the autonomic nervous system are inseparable. Sleep plays a key role in circadian regulation of autonomic function, and the autonomic nervous system is essential for switching between sleep stages, maintaining the airway, and stabilizing the cardiovascular system. When this two-way relationship is destroyed, a vicious cycle is formed in which sleep disorders and autonomic dysfunction mutually worsen.
Normal sleep and autonomic nervous system
NREM sleep
In NREM sleep (stages 1 to 3), parasympathetic nerves become dominant and sympathetic nerve activity gradually decreases. In slow-wave sleep (N3), sympathetic nerve activity decreases by more than 50% compared to daytime, and heart rate, blood pressure, and respiratory rate are lowest. This is the cardiovascular system's "nocturnal recovery" time.
REM sleep
In REM sleep, autonomic nerve activity becomes unstable. Intermittent sympathetic bursts occur, and heart rate and blood pressure fluctuations increase. During the 'phasic REM' period, when sympathetic activity surges in synchronization with rapid eye movements, the risk of arrhythmia temporarily increases.
Nocturnal dipping
In normal people, blood pressure during sleep decreases by 10-20% compared to during the day. This is called ‘dipping’ and is the result of parasympathetic dominance and sympathetic inhibition. The 'non-dipping' pattern of loss of nocturnal blood pressure drop suggests autonomic dysfunction and is an independent risk factor for cardiovascular disease.
Sleep apnea and autonomic nervous system
sympathetic nerve hyperactivity
Obstructive sleep apnea (OSA) repeats the cycle of repetitive upper airway obstruction → hypoxia → hypercapnia → microarousal. With each apnea episode, the sympathetic nervous system is rapidly activated, causing blood pressure to spike and heart rate to fluctuate.
In severe OSA (AHI ≥ 30), nocturnal muscle sympathetic nerve activity (MSNA) is approximately twice that of normal people, and this sympathetic hyperactivity not only occurs during sleep but also persists during the day.
cardiovascular outcomes
There is a large-scale observational study showing that the risk of fatal and non-fatal cardiovascular events is approximately 2.87 times higher in the untreated severe group of OSA patients. It is an independent risk factor for daytime hypertension, nocturnal non-dipping, atrial fibrillation, and heart failure.
Autonomic effects of CPAP treatment
Continuous positive airway pressure (CPAP) treatment has been reported to significantly reduce nocturnal sympathetic hyperactivity, lower blood pressure by approximately 2 to 3 mmHg, and restore nocturnal dipping patterns. In the long term, the incidence of cardiovascular events also decreases.
Insomnia and autonomic nervous system
Hyperarousal Hypothesis
The core pathophysiology of chronic insomnia is a hyperarousal state of the central nervous system. Sympathetic nerve activity increases throughout the 24 hours, and this manifests itself as fatigue, difficulty concentrating, and increased resting heart rate not only at night but also during the day.
HRV changes
When analyzing nighttime HRV in insomnia patients, a sympathetic dominance pattern is observed with a decrease in the HF (parasympathetic) component and an increase in the LF/HF ratio. This means that sufficient parasympathetic activation is not achieved even during sleep.
therapeutic approach
Cognitive behavioral therapy for insomnia (CBT-I) relieves hyperarousal through sleep restriction, stimulus control, and relaxation training, and has been reported to improve HRV. During drug treatment, melatonin receptor agonists are more beneficial for autonomic balance than benzodiazepines.
REM sleep behavior disorder and autonomic nervous system
REM sleep behavior disorder (RBD) is a sleep disorder in which muscle tone inhibition is lost during REM sleep and behavior is consistent with dream content. More than 80% of idiopathic RBD patients convert to alpha-synuclein diseases such as Parkinson's disease and multiple system atrophy within 10 to 15 years.
Patients with RBD are often accompanied by autonomic dysfunction such as orthostatic hypotension, decreased HRV, and abnormal sweating even before the disease transition, which is interpreted as a precursor sign of neurodegeneration.
Sleep problems in autonomic dysfunction
Orthostatic hypotension and sleep
Paradoxically, orthostatic hypotension, which occurs in pure autonomic failure and multiple system atrophy, is often accompanied by nocturnal supine hypertension. Nocturnal hypertension causes pressure diuresis, which increases nocturia and worsens sleep fragmentation.
POTS and sleep
67-93% of POTS patients complain of sleep disorders, and difficulty falling asleep, difficulty maintaining sleep, and excessive daytime sleepiness are common. Sympathetic hyperactivity persists even at night, deteriorating sleep quality.
diagnostic evaluation
- Nocturnal polysomnography (PSG): Simultaneous assessment of sleep architecture, respiratory events, and heart rate fluctuations
- Nighttime HRV analysis: evaluation of autonomic function by sleep stage
- 24-hour ambulatory blood pressure measurement (ABPM): Check nighttime dipping pattern
- Autonomic nerve test battery: standing tilt test, Valsalva maneuver, QSART, etc.
therapeutic approach
Treatment of sleep disorders and autonomic dysfunction requires an integrated approach that addresses both simultaneously. CPAP treatment for OSA, CBT-I for insomnia, and treatment of the cause of autonomic dysfunction are key, and sleep hygiene education and regular exercise play a supporting role.