Reducing screen brightness at night on Linux with xcalib

xcalib (debian package) has saved my eyes quite a few times on long nights. It can reduce effective screen brightness below minimum settings by altering the contrast. I usually reduce the contrast to 70% in a totally dark room:
xcalib -a -co 70
The -a option is to alter the current setting. -co is the contrast level in percentage.

Another fun thing it can do for night reading is color inversion with the -i option:
xcalib -a -i
To undo inversion, just run that command again to invert the inversion.

To reset changes made with xcalib, use the -c option to get back to normal:
xcalib -c