I try to write a simple 2d game using python 3.x and pygame 1.9.2. I want to simulate changing of days/nights. I have got in-game time(like real, but faster) and I'd like to make my screen darker at exact time but have no ideas how to do it. Could you advice me how to reduce the brightness of my screen or other ways how to show changing of days and nights?
Excuse for my language if I make mistakes. English isn't native for me)
Setting gamma or screen brightness is not the best way to go. 
When your game gets more complicated, it's easier if you use the technique ecline6 suggested:
changing your background image.
But since it is a loaded image i suppose, you can use this trick:
Let's say you have a background surface. Every 10s you will fire an event that will change the backround by blitting an low alpha-black surface on top of it. That way your background will get darker.
You can do the same with white, to make it more brighter too.
Changing days and nights with pygame