Here is a conversation I have had when helping people with their mobile devices:
Me: Your screen is quite dark. Do you like it this way?
Client: No, not really.
Me: May I show you how to make it brighter?
Client: I know how, but I keep it dark on purpose.
Client: Because the darker screen setting saves battery power.
I implore you, stop this madness!
Does your battery really run out before the end of the day? If you must save battery power, there are better ways than squinting: turn off wifi if you are not near your network, turn off notifications and location services, don’t automatically update apps, fetch mail less often, etc. Or get a case with a built-in charger.
Use your battery; save your eyesight!