Led lights

After a workout ends, they stay on for a few minutes to show how your heart rate slows down.

Also (if you wish), during running and walking workouts, turn on Power Saving Mode to disable the heart rate sensor. To do this, open the Apple Watch app on iPhone, go to My Watch > Workout, and turn on Power Saving Mode. Note that when the heart rate sensor is off, calorie burn calculations may not be as accurate.

They will shot off when they are no longer being used. They are not user controllable.