So yeah... they patched the gamma 4 - gamma 2 console thingy, because they thought it interfered with the game difficulty.
So here's how you can still easily increase your gamma in Win 7 / Win 8 without installing additional software.
Note: this may not work for everyone, not sure why yet.
EDIT: it seems you may have to run your game in Windowed or Windowed Fullscreen for this to work.
Open Start, and type "calibrate", then choose Calibrate display color.
You'll get this window. Click next twice.
Now you can change your gamma.
During night, I drag the slider all the way up. During the day, I put it to about 50% (where it is by default).
Enjoy!