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Autonomic Nervous System | What Is ANS Charge?

When you wake up every morning and check your sleep with Nightly Recharge™, do you ever wonder what your ‘ANS charge’ means? Of course, you can see that your Polar smartwatch measures your heart rate, breathing rate, and many other functions while you sleep, but what does ANS stand for exactly? How do these measurements indicate the quality of your sleep? Well, it’s time we all became more familiar with our autonomic nervous system and the essential role it plays when we’re asleep (and awake).

What is the autonomic nervous system?

Your autonomic nervous system forms part of your peripheral nervous system (PNS), which encompasses all the nerves that lie outside your central nervous system (CNS). Your PNS is responsible for connecting your CNS to all your limbs, organs, and skin, which means this nervous system runs all the way out to your extremities.

All your automatic body functions, such as breathing and your heartbeat, are regulated by your autonomic nervous system (ANS), also known as your involuntary nervous system. We’re rarely conscious of these processes taking place in our bodies, yet they often run like clockwork, hence why our ANS is so essential. It regulates some pretty essential functions, including:

  • Body temperature
  • Production of sweat, saliva, and other body fluids
  • Digestion
  • Metabolism
  • Electrolyte balance
  • Heart rate
  • Blood pressure
  • Respiratory rate (breathing)
  • Pupillary response (eyes)
  • Sexual responses
  • Urination and defecation
  • Emotional responses


Many of the above functions are regulated by three separate divisions of the ANS. These are your:

  • Sympathetic nervous system, which handles your rapid involuntary response to stressful or dangerous situations. So, when your heart rate speeds up, your pupils dilate, and you experience a ‘fight or flight’ response, this nervous system has been activated.
  • Parasympathetic nervous system, which returns your body to its ‘regular’ functioning after a stressful situation. It’s also responsible for all your ‘rest and digest’ functions, such as decreased heart rate and increased production of saliva and mucus.
  • Enteric nervous system, also known as your intrinsic nervous system, controls gastrointestinal behavior through a mesh-like network of neurons within the walls of the digestive organs. 

Sleep and your autonomic nervous system

So, what does sleep and our autonomic nervous system have to do with each other? Well, sleep is a time when our body’s parasympathetic nervous system is activated to help us rest and recover, diminishing our sympathetic nervous system response. This rebalancing allows our body to replenish physiologically, ensuring we are better prepared for the following day.

When we struggle to relax and still our minds at night, we can have trouble falling asleep. Our heart may race, our thoughts make us stressed, and we might get little to no sleep. When this happens, our sympathetic nervous system struggles to achieve its natural balance with our parasympathetic nervous system. Having trouble sleeping happens to us all occasionally and shouldn’t be a source of worry. 

However, suppose this happens to you regularly. In that case, this overactivity of the mind and restlessness of the body can mean you aren’t recovering each night properly and may be struggling with insomnia. You can help your sympathetic nervous system relax in the evenings by creating the right environment for your sleep, such as reducing light and noise in your sleep space. You can also try a sleep mediation, which has been shown to decrease the level of stress hormones in your body and allow you to fall asleep easier.

What is Polar’s ANS Charge on Nightly Recharge™?

Autonomic Nervous System | What Is ANS Charge?

When you check your Nightly Recharge™ status each morning, you’ll see that it tracks two separate metrics – ANS charge and sleep charge. The results from both of these are combined to give you a Nightly Recharge rating (such as Good or Compromised), so you can make an informed decision about how much you exert yourself each day. Based on your Nightly Recharge™ status, Polar also provides personalized daily tips on exercise, sleep, and regulating energy levels.

Sleep charge is relatively straightforward, measuring the amount and quality of your sleep each night. It is a classic sleep metric, looking at the hours of rest and your sleep cycles, then comparing them to your previous 28 days to see how well you rested. ANS charge is a little different, which is why the combination of these two scores makes Polar’s Nightly Recharge™ is so comprehensive. Think of them as a sleep quantity and a sleep quality measurement. 

ANS charge measures how well your autonomic nervous system calmed down during the first hours of sleep compared to your usual level over the previous 28 days. By measuring your heart rate, heart rate variability, and breathing rate via Polar’s optical sensor on your wrist, your device can calculate your ANS charge. These measurements are only taken for around the first four hours of your sleep to see how your system relaxes.

Heart rate has the most significant influence when forming ANS charge and breathing rate the smallest. The higher the ANS charge, the better your cardio system has recovered. Increased heart rate and decreased heart rate variability are considered to indicate poor recovery. In contrast, reduced heart rate and increased heart rate variability are considered to show good recovery.

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Please note that the information provided in the Polar Blog articles cannot replace individual advice from health professionals. Please consult your physician before starting a new fitness program.

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