Read on to learn how the sympathetic and parasympathetic systems differ, how too much sympathetic nerve activity contributes to chronic diseases, and how to trigger the rest-and-digest state.
You’ve heard of “fight or flight,” but what about its counterpart, “rest and digest”? Check out this article to learn more about chronic stress and fight-or-flight mode and get tips on how to let your body rest and digest.
The Difference between “Fight or Flight” and “Rest and Digest”
The sympathetic nervous system (SNS) and parasympathetic nervous system (PNS) are two of the three components of the autonomic nervous system, which controls unconscious body functions—like breathing and digestion—as well as every organ in our bodies except for skeletal muscles.
You might remember from Biology 101 that the SNS is responsible for the fight-or-flight response to a stressor or danger, while the parasympathetic system controls the rest-and-digest functions of the body. Here’s a brief synopsis: (1)
The sympathetic nervous system prepares your body to either run from danger or fight back. It’s also activated in response to mental or physical stress. During the fight-or-flight response, the following occurs:
- Blood pressure increases
- Blood flow increases to muscles, lungs, and other areas essential for moving away from perceived danger
- Blood flow decreases to the digestive and reproductive systems
- Stress hormones, such as cortisol, and neurotransmitters, like epinephrine, increase to make us stronger and faster
- Glucose is rapidly released to be burned for quick energy
The parasympathetic nervous system is activated after a meal or in response to pleasure, and its physical effects are generally opposite those of the SNS reaction:
- Heart rate and respiration slow
- Blood pressure drops
- Intestinal activity increases
- Blood flow increases to the digestive tract
- Neurotransmitters like acetylcholine, which regulates muscle contractions, including cardiac muscle, are released
- Stress hormones decrease
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Sympathetic vs. Parasympathetic: It’s All about Balance
The sympathetic nervous system interacts with the hypothalamus–pituitary–adrenal (HPA) axis to control the body’s stress response. (2) Stress triggers the adrenal glands to secrete hormones, including cortisol, epinephrine, and norepinephrine, in order to increase blood pressure and blood sugar. Following a stressful event, the parasympathetic nervous system should kick in to decrease stress hormone production and lower blood pressure through the release of the neurotransmitter acetylcholine, among others.
To say the SNS and PNS are antagonistic is an oversimplification. The two systems can work with each other, against each other, or even independently of one another in order to maintain homeostasis, or equilibrium throughout the body’s systems. (3) It’s all about balance.
Although their lives weren’t stress-free by any means, our ancestors knew how to rest and recharge. Embracing their lifestyle and behavior now, as modern humans, is the driver behind ancestral health.
Chronic Stress, Autonomic Balance, and Heart Rate Variability
In contrast with our ancestors, we lead lives often filled with distractions and busyness. Stress can be persistent—you might constantly worry about money or a bad relationship, or experience continuous stress from a job that leaves you connected to your smartphone 24/7, or consistently get fewer than six hours of sleep each night, all of which will continually trigger the sympathetic nervous system’s fight-or-flight response. Most likely, you’re one of the eight in 10 Americans who, according to a 2017 Gallup poll, say that they are stressed. (4)
If you’re suffering from chronic stress, you might experience the following symptoms:
- Decreased immunity
- Sleep problems
- Mood swings
- Sugar and caffeine cravings
- Irritability or lightheadedness between meals
- Eating to relieve fatigue
- Dizziness when moving from sitting or lying to standing
- Digestive distress
Chronic stress upsets the balance between the parasympathetic and sympathetic nervous systems.
When you’re chronically in fight-or-flight mode, being in that “on” state disrupts the body’s stress response system by raising overall cortisol levels and disturbing normal cortisol and melatonin rhythms. (5, 6, 7)
A good measure of parasympathetic–sympathetic balance is heart rate variability. (8) Unlike heart rate, which reports the average number of beats per minute, heart rate variability measures the subtle variations in the time interval between individual beats. Although some wearable health trackers can’t measure heart rate variability, more sophisticated ones like the Oura ring can.
In general, increased heart rate variability indicates better health and better autonomic balance between the PNS and SNS. Heart rate variability is often referred to as vagal tone, named after the vagus nerve—the longest nerve in the body—which comprises 75 percent of the entire parasympathetic nervous system. (9) The vagus nerve connects to the heart, lungs, and digestive tract. In fact, the vagus nerve is a critical component of the gut–brain axis, as a direct line of communication between the gut microbiome and the brain. (10)
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Eight Effects Stress Causes in Your Body
Our bodies aren’t meant to withstand constant stress. An overactive SNS disrupts the HPA axis, induces chronic inflammation, and ultimately has been linked to a number of chronic health conditions:
1. Cardiovascular Disease
Chronic sympathetic nerve activity increases blood pressure and destructively changes the shape, size, and organization of blood vessels. (11, 12) In one study of heart failure patients, sympathetic nerve activity was an independent predictor of one-year cardiac mortality. (13)
2. Insulin Resistance and Type 2 Diabetes
To provide quick energy to run from a predator, the body’s fight-or-flight response increases glucose blood levels, and chronically elevated levels can lead to insulin resistance. (14) Those with type 2 diabetes often demonstrate sympathetic hyperactivity. (15, 16)
4. Weakened Immune System
Both the SNS and PNS regulate the immune system in various ways. High cortisol from continuous SNS stimulation can suppress immune function, and an overactive SNS may shift the body’s immune balance to be susceptible to autoimmune conditions like rheumatoid arthritis. (20, 21, 22)
5. Kidney Disease
6. Depression, Anxiety, and Other Mood Imbalances
Because of parasympathetic nerve action on the vagus nerve, an underactive PNS may contribute to mood imbalances. Also, the autonomic nervous system mediates inflammation and the immune response, both of which are implicated in depression, anxiety, and more. (26, 27, 28)
7. Ulcerative Colitis, Crohn’s Disease, and Irritable Bowel Syndrome
We know stress impacts the gut and that chronic stress leads to SNS overactivity. An imbalance between the SNS and PNS has also been implicated in ulcerative colitis, Crohn’s disease, irritable bowel syndrome, and other gut issues. (29, 30, 31)
8. Poor Sleep
It’s a vicious cycle: stress can lead to poor sleep, which leads to more stress and more disruption of natural melatonin rhythms. Sleep problems are implicated in obesity, insulin resistance, heart disease, impaired cognitive function, and more. (32, 33, 34, 35, 36, 37).
12 Ways to Trigger Rest and Digest
You may be able to get acute stress under control pretty easily (especially if you make a point of managing it), and everyday stress often just passes with time (the rush hour traffic will end, you’ll turn in the project, the “big day” on the calendar comes and goes). But if you’re battling chronic stress over an extended period of time, your sympathetic and parasympathetic nervous systems are probably out of balance. Try some of the following activities and techniques to start triggering rest and digest on a regular basis:
- Deep breathing: Breathing exercises can increase vagal tone and facilitate digestive motility. (38) Check out my favorite breathing techniques to get started.
- Meditation: Meditation, especially mindfulness meditation, has been shown to increase PNS activity, decrease SNS activity, and improve heart rate variability. (39, 40, 41, 42)
- Massage therapy: Whether it’s a head massage, hand massage, or full-body deep tissue massage, this type of touch can improve the PNS as measured by heart rate variability. (43, 44, 45, 46)
- Media fast: Smartphone use can increase heart rate and decrease autonomic nervous system activity. (47, 48) Regularly taking breaks from tablets, phones, and computers can help keep us balanced and focused.
- Acupuncture: When performed by a licensed professional, acupuncture can increase vagal tone and reduce heart rate and inflammation. (49)
- Exercise: The sympathetic nervous system is activated during exercise, but regular exercise correlates with an increase in resting vagal tone and overall health. (50)
- Yoga: Yoga increases vagal activity by combining facets of exercise, meditation, and deep breathing. (51)
- Nature: Regular interaction with nature improves autonomic tone and heart rate variability. (52)
- Pleasure: Pleasure looks different for different people depending on hobbies and interests, but it’s not the same as distractions like social media and binge watching a TV series.
- Cold-water face immersion: Especially after strenuous exercise, splashing your face with cold water can jumpstart the parasympathetic nervous system. (53, 54, 55)
- Yawning: The exaggerated inhale and exhale of a yawn triggers the PNS, probably in a similar way to controlled breathing exercises. (56, 57)
- Gum chewing: Chewing stimulates digestion, controlled mostly by the parasympathetic nervous system. Chewing gum does the same. (58)
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