Copy/Paste from FB group. It is not mine. I'm sorry if it has been mentioned again.
INFO from Garmin Support Center about wrist HR SENSOR and its ACCURACY (usable information)
"Thank you for contacting Garmin Iberia.
In order to solve your query, we will detail the functionality of the sensor that incorporates your watch.
The heart rate monitor that incorporates your device through the wrist is a useful tool that can provide an accurate estimate of the user's heart rate at any given time. The FC optical monitor is designed to try to monitor the user's heart rate 24 hours a day, 7 days a week. The frequency with which the heart rate is measured varies and depends on the level of activity of the user. When you start an activity with the Garmin FC optical device, the FC monitor provides more frequent information, since the optical sensor is activated at all times and attempts to continuously measure the heart rate during a given period of activity. The purpose is to provide the user with a heart rate reading with greater frequency and accuracy during a given activity.
Although our HR monitor includes state-of-the-art technology, there are inherent limitations to the technology that may cause some of the heart rate readings to be inaccurate depending on certain circumstances. These circumstances include the physical characteristics of the user, the setting of the device and the type and intensity of the activity, as described above. FC monitor data should not be used for medical purposes or to diagnose, treat, cure or prevent any disease or condition.
So, from Garmin, we recommend using the internal optical sensor for wellness control, but for training we recommend the use of a chest band - check compatibility - the measurements will always be more accurate with the chest band, optical technology has its limitations .
Regards / Best Regards"