Apple Watch Series 1 - Screen light is very dim - how to fix this?

My apple watch - series 1 was purchased almost 3 years ago and it gave me no problem till date. However in the past one week, the screen has suddenly become very dim. When the hand is raised, the screen has full brightness, however this is momentary and within a few seconds the screen is very dim, barely enough to read the time on display.

Any suggestions how to fix this?

P.S: I have restarted and repaired both the phone and apple watch.

Both the devices have the latest software updates.

I use an iPhone 6 (March 2015) and Apple Watch Series 1 (Dec 2015)