Well specifically to the brightness, how much light is going into your eyes when using a VR set relative to, say, a TV? Or a 27-inch monitor close to your face?
Yep, I really regret spensing so much time of my youth indoors in front of a monitor... At age 19 I can experience the loss in sight quality already. It is recommended to spend AT LEAST 2 hours everyday outside in daylight to prevent myopia. I did not follow that recommendation and regret it sorely.
I’d imagine most discomfort is eye strain rather than brightness. The way VR headsets work right now it’s important to have your eye correctly positioned and the headset adjusted for your own distance between pupils.