I wear a chest strap heart rate monitor when running year round, because the native optical tracking on even the best sports watches is often unreliable. The wrist is a tough place to get an accurate reading during workouts, so the best heart rate monitors are all chest straps like the Polar H10.
The accuracy of optical sensors only gets worse in cold weather, so if you’ve noticed that the readings from your running watch has become increasingly erratic of late, that may be why.
