Heartline Study Apple Watch

Several times during the day's wearing of my Apple watch, especially during my morning workouts or during the days where I may be very physically active, the brightness of the watch face will change from being bright and readable to being so dark that one can faintly make out the readings. I have set the brightness scale at the top end but this still happens. Then, for no certain pattern or events, the brightness may appear normally on the face readouts. I have spoken with other wearers of the Apple watch and they have not experienced this happening. I would appreciate your response of this annoying and disturbing event.