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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 you became more familiar with your autonomic nervous system and the essential role it plays when you’re asleep (and awake).

What is the autonomic nervous system?

Your autonomic (pronounced “auto-nom-ick”) 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 crucial 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. It can function independently to control digestive activities, which is why it’s sometimes called the second brain. 

Sleep and your autonomic nervous system

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

When you struggle to relax and still your mind at night, you can have trouble falling asleep. Your heart may race, your thoughts could make you stressed, and you might get little-to-no sleep. When this happens, your sympathetic nervous system struggles to achieve its natural balance with your 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 meditating, 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 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 and 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™ 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. It also compares it to your usual level over the previous 28 days. It is by measuring your heart rate, heart rate variability, and breathing rate via Polar’s optical sensor on your wrist that your device can calculate your ANS charge. It’s important to note that these measurements are only taken during the first four hours of sleep to see how your system relaxes. That’s why drinking alcohol or eating a large meal close to your bedtime can affect your ANS charge because your body hasn’t had time to process them before you sleep and, therefore, is not relaxed.

Heart rate has the most significant influence when forming ANS charge, and breathing rate is 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 supposed 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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