Your heart beats, your food digests and your pupils adjust to light without you ever thinking about it. That is the work of the autonomic nervous system, and two of its branches, the sympathetic and parasympathetic nervous systems, pull in opposite directions to shape how you feel physically and emotionally. Knowing how they work makes it easier to understand stress, and what you can do about it.
What is the autonomic nervous system?
The autonomic nervous system regulates involuntary processes such as heart rate, blood pressure, breathing, digestion and sexual arousal. It has three divisions: the sympathetic nervous system, the parasympathetic nervous system and the enteric nervous system, a web of more than 100 million neurons in the gut wall that largely runs digestion on its own. The sympathetic and parasympathetic branches act like an accelerator and a brake, and most organs receive input from both.
What does the sympathetic nervous system do?
The sympathetic nervous system drives the "fight or flight" response. It is what makes you jump back from a spider before you have consciously registered it. When it fires, sympathetic nerves release norepinephrine and the adrenal glands release adrenaline, which:
- raise heart rate and blood pressure
- widen the pupils and open the airways
- release stored glucose for quick fuel
- slow digestion, since it is not needed to escape a threat
That response kept our ancestors alive when the threat was a predator. Today it switches on just as readily for a tense meeting, a pile of unread email or a scary news alert. Your pupils dilate, your breathing quickens, your mouth goes dry and your heart pounds, even though there is nothing to run from.
What does the parasympathetic nervous system do?
The parasympathetic nervous system handles "rest and digest." It slows the heart, restores the muscular contractions that move food through the gut, and supports the functions that let the body repair and reproduce, including:
- saliva production
- release of digestive juices
- peristalsis, the wave-like movement of the intestines
- blood flow to the sexual organs
When the parasympathetic side is in charge, you yawn, stretch, laugh and breathe slowly from the belly. Ideally the two systems trade places through the day: a burst of sympathetic activity when you need it, followed by a return to rest.
Why does modern life keep us in fight-or-flight?
Many everyday habits nudge the balance toward the sympathetic side:
- Coffee. In a controlled study, drinking coffee acutely increased sympathetic nerve activity in both habitual and occasional coffee drinkers, though only occasional drinkers saw a rise in blood pressure.
- Short sleep. When healthy young men were limited to four hours in bed for six nights, their evening cortisol and sympathetic nervous system activity rose. More than a third of US adults usually sleep less than seven hours a night, according to CDC data.
- Shallow breathing. Hunching over screens and breathing fast and high in the chest is the breathing pattern of stress, not rest.
- Constant low-grade stress. Work, money, relationships and the news can keep the stress response switched on long after the moment has passed.
How can you activate your parasympathetic nervous system?
Most of the best tools are free.
- Slow your breathing. A systematic review found that slow breathing, below about 10 breaths a minute, is linked with greater heart rate variability and feelings of relaxation and wellbeing. Try breathing in for four counts and out for six for a few minutes.
- Protect your sleep. Aim for at least seven hours, with a regular bedtime. Read more in our article on sleep deprivation.
- Watch the caffeine. If you feel wired or anxious, try cutting back or keeping coffee to the morning.
- Build in recovery. Walks outdoors, yoga, mindfulness, a warm bath and time with people you love all help the body shift back into rest.
- Eat well. Regular, balanced meals help avoid the stress of blood sugar swings. Stress and digestion are closely linked, too; see how chronic stress affects gut health.
Can adaptogens help with stress?
If you have been running on stress, caffeine and short sleep for years, shifting back into "rest and digest" can take time. Adaptogenic herbs such as ashwagandha are traditionally used to help the body cope with stress, and a 2022 meta-analysis of 12 trials found ashwagandha reduced stress and anxiety scores compared with placebo, although the certainty of the evidence was rated low.
Stress Release combines five clinically studied plant extracts, including Sensoril ashwagandha and Affron saffron, to support stress management and relaxation without drowsiness. Ashwagandha is not for everyone: the NIH advises avoiding it during pregnancy and breastfeeding and if you have a thyroid or autoimmune condition, and it may interact with some medications, so check with your doctor first. Learn more in what are adaptogens? and ashwagandha benefits for women.
