Sleep and the Autonomic Nervous System: How 'Enjoyment' Regulates the Parasympathetic Nervous System
"I'm tired, but for some reason I can't sleep," "My brain keeps running even after I get into bed," "I'm sleeping, but I don't feel well-rested"—many entrepreneurs and business owners raising children struggle with these issues.
At the root of this is a "failure to switch" in the autonomic nervous system.
After running in "combat mode" (sympathetic nervous system dominance) all day, the brain and body fail to switch to relaxation mode (parasympathetic nervous system dominance) even after getting into bed. If this switch doesn't happen, you cannot enter deep sleep no matter how long you keep your eyes closed.
And one of the most natural ways to flip this switch is to "enjoy" yourself. Today, I will carefully explain the mechanism by which enjoying yourself regulates the parasympathetic nervous system and deepens sleep, from the perspectives of neuroscience and the autonomic nervous system.
◎ The true nature of sleepless nights is a "failure to switch" in the autonomic nervous system
☆ What is the autonomic nervous system? A story of the accelerator and the brake
The autonomic nervous system is the nervous system that regulates functions the body performs automatically without conscious effort, such as heartbeat, breathing, digestion, and body temperature regulation. "Autonomic" means that you do not need to control it consciously.
There are two main types of this autonomic nervous system: the sympathetic nervous system and the parasympathetic nervous system.
The sympathetic nervous system can be compared to an "accelerator." It becomes dominant in states where you are "moving toward something," such as activity, tension, concentration, fighting, or fleeing. Heart rate increases, blood pressure rises, and blood flow is directed to the muscles. It is designed to function actively from morning through the day.
The parasympathetic nervous system can be compared to a "brake." It becomes dominant in states of "resting and regulating," such as rest, recovery, digestion, and relaxation. Heart rate calms down, blood pressure drops, and digestion is promoted. It is designed to become dominant from evening through the night.
In a healthy state, the rhythm of sympathetic dominance during the day and parasympathetic dominance at night functions properly. However, in modern life, especially for entrepreneurs and business owners balancing child-rearing and work, this rhythm is easily disrupted.
☆ What happens when you go to bed while the sympathetic nervous system is dominant?
A state of sympathetic dominance is one where the brain judges that "now is the time to be active." If this state continues, the brain cannot receive the "instruction to rest" even after you get into bed.
Specifically, cortisol (the stress hormone) remains high. Cortisol is a hormone that promotes wakefulness, and if it remains high at night, it becomes difficult to enter deep non-REM sleep. The secretion of melatonin (the sleep hormone) is also easily suppressed when cortisol is high. This leads to a state where sleepiness is hard to come by, and even if it does, you wake up quickly.
Also, in a state of sympathetic dominance, Heart Rate Variability (HRV) decreases. HRV is the fluctuation in the intervals between heartbeats and is an indicator that reflects the activity of the parasympathetic nervous system. Research shows that higher nighttime HRV correlates with better sleep quality. When HRV is low—meaning the parasympathetic nervous system is not functioning sufficiently—it becomes difficult to obtain deep sleep.
☆ Why the modern "day full of obligations" hinders the switch
A sense of obligation, such as "I have to do this," is processed by the brain as a "threat signal." "I won't make the deadline," "I haven't finished that work yet," "I need to be more proper"—as long as these thoughts continue, the sympathetic nervous system continues to judge that you are "still in combat."
The problem is that the "threat of obligation" is hard to end. After fleeing from an external enemy, the threat is gone, so cortisol levels drop. But the threat of "things I need to do are not finished" can continue all day long.
In this way, the sympathetic nervous system remains dominant even at night, and the switch to the parasympathetic nervous system does not occur.This is the structure that creates nights where you are 'tired but cannot sleep'.
◎ Brain science conditions for making the parasympathetic nervous system dominant
☆ 'Safety' signals activate the parasympathetic nervous system
What is necessary for the parasympathetic nervous system to become dominant? The answer is 'feeling safe'.
The brain only switches to full relaxation mode when it judges that 'it is safe now.' Conversely, in a state where one thinks 'it might still be dangerous' or 'there is still an unresolved problem,' the switch to the parasympathetic nervous system does not occur.
Where do these 'safety signals' come from? In terms of brain science, information that 'it is safe now' reaches the brain through various channels, such as bodily sensations, breathing, facial expressions, tone of voice, and relationships with people around us. Being in a smile, breathing slowly and deeply, and enjoying oneself—these function as 'safety signals' to the brain.
☆ Polyvagal Theory: The nervous system has three states
The Polyvagal Theory, proposed by American neuroscientist Dr. Stephen Porges in 1994, significantly rewrote our understanding of the autonomic nervous system (Source: Porges SW, 1994, Psychophysiology).
Previously, the autonomic nervous system was understood through a dichotomy of 'sympathetic vs. parasympathetic,' but Dr. Porges discovered that the vagus nerve, which is responsible for the parasympathetic nervous system, is further divided into two systems.
The first is the 'ventral vagal complex.' This is an evolutionarily new nervous system that becomes dominant when one feels 'safety, security, and connection.' It is activated when fostering social connections, feeling pleasant enjoyment, and laughing together.
The second is the 'dorsal vagal complex.' This is an evolutionarily old nervous system related to extreme defensive reactions of 'freezing, shutting down, and dissociation.'
The sympathetic nervous system is positioned in the middle as 'fight or flight' (fight-or-flight response).
In other words, the human nervous system has three stages of states: safety (ventral vagal dominance) → threat/combat (sympathetic dominance) → extreme crisis (dorsal vagal dominance). And,to enter deep sleep, it is necessary to enter the 'state where the ventral vagal complex is dominant' among these, which is the 'state of feeling safe and connected'.
☆ Why enjoying yourself becomes a 'safety signal'
The act of 'enjoying' is one of the most powerful 'safety signals' for the brain.
When you are purely enjoying yourself, the brain judges that 'I don't have to fight now,' 'I don't have to run away now,' and 'it is safe now.' Smiles, a soft voice, relaxation of the body, and pleasant conversations with someone—these are all signals that send information to the ventral vagal complex that 'it is safe now.'
From the perspective of Polyvagal Theory, enjoyment is not merely a 'distraction,' but an active measure that guides the nervous system into 'safety mode.' And it is this 'safety mode' that serves as the gateway to deep sleep.
◎ Four pathways through which 'enjoyment' regulates the parasympathetic nervous system
☆① Laughter and breathing: The diaphragm stimulates the vagus nerve
The act of 'enjoying and laughing' has a physiological pathway that directly activates the parasympathetic nervous system.
When you laugh, your diaphragm moves significantly. This movement of the diaphragm stimulates the vagus nerve (the primary nerve that accounts for approximately 80% of the parasympathetic nervous system). When the vagus nerve is stimulated, the parasympathetic nervous system becomes dominant, heart rate stabilizes, blood pressure drops, and the entire body moves toward relaxation.
Furthermore, the breathing pattern during laughter (inhaling and then producing laughter) promotes a phenomenon called respiratory sinus arrhythmia (RSA). RSA is the natural fluctuation where the heart rate increases when inhaling and slows down when exhaling, which is directly linked to high heart rate variability (HRV). Increased HRV is evidence that the parasympathetic nervous system is functioning adequately.
Research on the relationship between laughter and HRV has shown that spontaneous laughter (genuine laughter, not forced) increases HF power (high-frequency components), which is an indicator of the parasympathetic nervous system. Experiencing genuine laughter through enjoyment brings about physiological relaxation.
☆② Dopamine and the reward system: A 'satisfied brain' relaxes
Dopamine (the hormone of achievement, joy, and anticipation) secreted through enjoyment satisfies the brain's reward system.
When the reward system is sufficiently satisfied, the brain enters a state of 'that is enough for today.' This 'satisfaction signal' releases the maintenance of sympathetic nervous system arousal and encourages the transition to the parasympathetic nervous system.
Conversely, if you spend the whole day acting only out of a sense of duty and are dopamine-deficient, the brain's reward system continues to feel that 'something is still missing' and tries to maintain arousal. This dopamine deficiency is often the background to the state of 'being tired but unable to stop the mind.'
By enjoying yourself, dopamine is replenished, a feeling of 'today was enough' is created, and the brain naturally enters 'preparation for rest'—this is the second pathway through which enjoyment regulates the parasympathetic nervous system.
☆③ Oxytocin: Connection and enjoyment release the bonding hormone
Time spent enjoying yourself with someone, moments of laughing together, the moment you see the face of a beloved child—these 'feelings of connection' promote the secretion of oxytocin (the bonding hormone).
When oxytocin is secreted, the parasympathetic nervous system works dominantly, leading to a state where both mind and body are relaxed. Negative emotions such as anxiety and worry are eased, making it easier to feel peace and happiness.
Particularly noteworthy is the relationship between oxytocin and sleep. While oxytocin itself is not a hormone that directly induces sleepiness, it indirectly improves the quality of sleep by alleviating stress and anxiety, which are 'enemies of sleep' (Source: Churchland PS & Winkielman P, 2012, Neuron). It can be said that the time spent sharing enjoyable things with someone builds the foundation for sleep.
The reason why a nighttime routine of lying in a row with your children and talking about 'what was fun today' has more meaning than just a habit is because of this function of oxytocin.
☆④ Beta-Endorphins: How Laughter and Inspiration Create a Sedative Effect
Experiences such as laughter, inspiration, and joy trigger the secretion of a substance called beta-endorphin. Often referred to as the brain's 'natural opioids,' beta-endorphins possess powerful analgesic and sedative properties.
Beta-endorphins relax the body's tension, soothe pain and anxiety, and function as a signal to the entire body that it is 'time to rest.' This sedative effect helps facilitate the transition into deep non-REM sleep.
It is said that the secretion of beta-endorphins caused by laughter has a sedative effect so potent that it shares a chemical structure similar to morphine. The feeling of being 'vaguely hazy and sleepy' after enjoying a good laugh is precisely the work of beta-endorphins.
◎ The Damage Caused to Sleep by Autonomic Nervous System Imbalance
☆ HRV (Heart Rate Variability) and Sleep Quality
Heart Rate Variability (HRV) is one of the most reliable indicators for measuring the health of the autonomic nervous system. While the heart may appear to beat at a constant rhythm, the intervals between beats actually fluctuate subtly. The greater this fluctuation (higher HRV), the better the parasympathetic nervous system is functioning.
Research shows that a higher average nighttime HRV correlates with better sleep quality and an increased proportion of deep non-REM sleep.Conversely, when HRV is low (meaning the parasympathetic nervous system is not functioning sufficiently), sleep tends to be shallow, one is more likely to wake up during the night, and there is a lower sense of recovery the next morning.
The physical sensation that 'the next morning feels different after a day spent enjoying oneself versus a day filled with obligations' can be scientifically explained by this difference in HRV. On days spent enjoying yourself, the parasympathetic nervous system functions well, HRV rises, and deep non-REM sleep increases. On days filled only with obligations, HRV remains low, and sleep ends up being shallow.
☆ What Happens During Sympathetic-Dominant Sleep
What exactly happens if you fall asleep while the sympathetic nervous system is dominant (or if it remains overactive during sleep)?
First, the efficiency of the glymphatic system (the system that removes waste from the brain during sleep) drops. This system functions most actively during deep non-REM sleep, but because sympathetic dominance reduces deep non-REM sleep, the brain's 'deep cleaning' becomes insufficient.
Second, the cortisol rhythm is disrupted. Normally, cortisol levels are at their lowest during sleep and rise sharply around the time of waking (the Cortisol Awakening Response, or CAR). When this rhythm functions normally, you wake up feeling refreshed. However, if nighttime cortisol levels are high, this rhythm is disrupted, leading to a 'heavy' feeling the next morning.
Furthermore, emotional processing becomes incomplete. The process of organizing the day's emotions occurs during REM sleep, but during sympathetic-dominant sleep, this processing is not performed sufficiently, making it easy to wake up feeling 'vaguely unsettled.'
☆ The Difference on Nights When the Parasympathetic Nervous System Works Well
In contrast, what is sleep like on nights when the parasympathetic nervous system functions sufficiently?
The proportion of deep non-REM sleep increases, allowing the glymphatic system to function fully. Brain waste products are efficiently removed, leaving you feeling refreshed the next morning.
Entering sleep with a high HRV increases the recovery efficiency of your entire sleep cycle. Even with the same six hours, six hours with a fully functioning parasympathetic nervous system makes a significant difference in fatigue recovery and brain repair.
Emotional processing also becomes more enriched. As shown in the aforementioned research from RIKEN and Kobe University (2025, Neuron), on days with positive emotional experiences, the coordination between the amygdala and the cerebral cortex during non-REM sleep increases, leading to deeper integration of emotions and memories.
The 'refreshed feeling,' 'lightness of mood,' and 'sense of looking forward to the day' the next morning are all products of sleep where the parasympathetic nervous system has worked effectively.
◎ Practices for using 'enjoyment' to reset the autonomic nervous system
☆ Designing a 'zero-obligation' 30 minutes before bed
To encourage the switch to the parasympathetic nervous system, try consciously designing the 30 minutes before you go to sleep.
The most important thing during this time is to 'make obligations zero.' Thoughts like 'I should do this' or 'I need to prepare for tomorrow' continue to stimulate the sympathetic nervous system. Use the 30 minutes before bed to distance yourself as much as possible from the 'I have to' mindset.
Effective ways to spend this time include listening to your favorite music and zoning out, watching your children sleep, burning your favorite aroma, or reading a few pages of a book you have been interested in. In all cases, it is important to act not out of 'I have to do this,' but out of 'I do this because I like it.' The moment your brain decides, 'It is safe now, it is okay to enjoy this,' the ventral vagal complex activates, and the switch to the parasympathetic nervous system begins.
☆ Stimulating the vagus nerve with laughter and deep breathing
There are two simple ways to directly stimulate the vagus nerve (the main pathway of the parasympathetic nervous system): laughter and deep breathing.
Regarding deep breathing, it is especially important to 'make the exhalation long.' Inhaling stimulates the sympathetic nervous system, while exhaling stimulates the parasympathetic nervous system. By inhaling for 4 seconds and exhaling slowly over 8 seconds—repeating this 3 to 5 times—the vagus nerve is stimulated, HRV increases, and the parasympathetic nervous system becomes dominant.
Regarding laughter, 'genuine laughter' is important. Forced laughter or obligatory smiles provide weak stimulation to the vagus nerve. As shown in the aforementioned research by Kramer & Leitao (2023, PLOS ONE), spontaneous laughter is the most effective at lowering cortisol and activating the parasympathetic nervous system.Even watching content that makes you laugh naturally for a short time before bed can induce physiological relaxation.
☆ Verbalizing 'what made me happy today' calms the nervous system
The habit of saying out loud one thing that made you happy or that you enjoyed today before going to sleep acts directly on resetting the autonomic nervous system.
Putting emotions into words activates the prefrontal cortex and calms the activity of the amygdala, which continues to process emotions excessively (Source: Lieberman MD et al., 2007, Psychological Science). Just by saying 'that made me happy' out loud, the overactivity of the amygdala is suppressed, and the switch from the sympathetic to the parasympathetic nervous system is encouraged.
Furthermore, by recalling the 'happy experience' you verbalized, oxytocin and dopamine are re-secreted, making it easier for the ventral vagal complex to transition into a state of 'feeling safe and connected'.
The brain immediately before falling asleep transitions into theta waves (a state of light sleep or dozing), which is the time when access to the subconscious is most open. By imbuing this 'golden window' with the feeling that 'today was enjoyable,' the nervous system can enter sleep while remaining in 'safety mode.'
Summary: The parasympathetic nervous system is activated by enjoyment.
Autonomic nervous system switching failure—this is the true nature of 'nights when you are tired but cannot sleep.'
A sympathetic-dominant state is maintained by a sense of duty, stress, and thoughts of 'I have to do this.' Switching to the parasympathetic nervous system occurs when a 'safety signal' reaches the brain. And enjoyment is one of the most natural 'safety signals' available.
Laughter stimulates the vagus nerve through the diaphragm (diaphragm-vagus nerve pathway), dopamine tells the brain that 'today was enough' (reward system pathway), oxytocin eases anxiety (bonding hormone pathway), and beta-endorphins relax physical tension (sedative pathway)—through these four pathways, enjoyment regulates the parasympathetic nervous system.
Three things you can try starting tonight:
- Make the 30 minutes before bed a 'zero-duty' time
- Take three deep breaths, inhaling for 4 seconds and exhaling for 8 seconds
- Say one thing you were happy about today out loud before falling asleep
As the Polyvagal Theory suggests, entering a state where the nervous system 'feels safe and connected' is the shortest route to deep sleep. And enjoyment is the most natural way to create that state.
Do not put off 'enjoyment'—this is not just a matter of lifestyle, but a neuroscientifically sound choice for regulating the autonomic nervous system and deepening your sleep.
■ Main References
Porges SW. The polyvagal theory: phylogenetic substrates of a social nervous system. International Journal of Psychophysiology. 2001;42(2):123-146.
Kramer CK, Leitao CB. Laughter as medicine: A systematic review and meta-analysis of interventional studies evaluating the impact of spontaneous laughter on cortisol levels. PLOS ONE. 2023;18(5):e0286260.
Lieberman MD, et al. Putting feelings into words: affect labeling disrupts amygdala activity in response to affective stimuli. Psychological Science. 2007;18(5):421-428.
Churchland PS, Winkielman P. Modulating social behavior with oxytocin: how does it work? What does it mean? Hormones and Behavior. 2012;61(3):392-399.
RIKEN/Kobe University Press Release: 'Neural mechanism by which emotion strengthens memory,' January 30, 2025.