I usually turned off all lights and played game in deep night after a whole-day work, even on bloody Quake I or Doom II, it is ok. And I was for a long time a Quake II fan, esp. in single player mode. With numerous add-on levels distributed on the web, I thought the 3D-shooter game has  reached the limit in Quake II. But alas, what a game the Half-Life is!  I  DARE NOT played it in the deep night alone with all lights off. With a very  good storyline and excellent atmosphere, the game is horrible!  You never  know what the next step will be. The monsters are not stupidly standing  there waiting for your bullets, instead they are chasing and hunting you.  It is simply the best single player 3d shoot game so far.