Why does my HRV often change throughout the day?

Users of our app or devices that continuously measure HRV throughout the day (such as Garmin watches or Oura rings) may notice that HRV often changes throughout the day.

The reason for this lies in the fact that the functioning of the heart in our body is regulated by the autonomic nervous system (ANS), which consists of the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). These two systems work continuously to maintain homeostasis (a state of balance) in our body.

What factors influence changes in HRV?

Here are examples of some factors that can influence your HRV within 5 minutes or even less.

Physical or psychological stress can reduce HRV

Physical or psychological stress can almost instantly trigger the sympathetic nervous system response (otherwise, humans wouldn’t have survived in the evolutionary process, for example, escaping from predators), mobilizing the body’s resources and reducing HRV.

When the stressor is no longer present, the body can compensate by increasing parasympathetic system activity to promote recovery.

Changes in breathing rate and depth of inhalation

Changes in breathing can quickly affect HRV. When breathing is slow and deep, parasympathetic nervous system (PNS) activity increases, which is why deep and slow breathing is often recommended for relaxation.

Changes in body position

Changes in body position (for example, quick standing up) also affect HRV as it causes a blood rush, requiring adjustments in the cardiovascular system.

Consuming caffeine or other stimulants

Caffeine intake can cause changes in HRV, and not immediately after drinking a cup of coffee, but over some time as caffeine is absorbed. We have a great article about caffeine and HRV.

What happens to HRV while we sleep?

It seems that we are in a calm environment, not experiencing stress, not drinking coffee, and the body is at rest. Shouldn’t HRV be the same in this case?

night hrv changes

Garmin Fenix watches, which measure HRV continuously during sleep, indicate that HRV constantly changes. But why does this happen?

Human sleep consists of several stages (deep sleep, rapid eye movement sleep, and slow-wave sleep), and the activity of the autonomic nervous system changes during these stages. Additionally, we may wake up during the night, and variations in room temperature can also elicit responses from the nervous system. These factors contribute to the continuous changes in HRV observed during sleep.


  1. The body’s nervous system constantly adapts to changes in the environment, leading to fluctuations in HRV. Our interest lies not just in these changes themselves but in the body’s ability to respond to stress.
  2. For obtaining more accurate results about the body’s functioning, it’s best to take measurements at the same time each day, preferably immediately after waking up. This allows the algorithm to establish a baseline state. Consistency is also crucial, so take measurements every day.
  3. If you notice that energy and stress parameters have changed within a few minutes, it’s normal. Your body is dynamic, so try taking measurements again after some time for a more comprehensive understanding.
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