I've played a bit over 3200 hours of Civ V. I often have trouble getting to sleep and playing Civ lets me do two things - get involved in the game when there are difficult decisions to make about what to build, what the better choice might be for a policy, or whether to take out a player, etc.; and during the mechanical, rote parts of play, lets my mind rove free to either work on some real life problems or to work on the wording of some writing. So, a few hours here, a few hours there, it adds up.
I'll try various mods, victory conditions, style of play, that kind of thing. Lastly, there are times that I'll just do something else and give Civ a rest - read, do chores, try out a new distro in a vm, tidy up twenty years of stuff on a handful of thoroughly disorganized hard drives, watch a movie, play another game - I've got Silent Hunter IV which can get challenging, and a few others; and maybe twenty or so I've haven't played yet, gotten at sales or the Humble Bundles.
To explain how I've got so much time, I have a very low income so don't get out much (and there's little I can afford to do when I do go out), am retired after years of health problems, and am still on crutches from last fall. I also read a lot - news, tech, and novels, along with a bit of history, biography, and philosoply; and I probably spend far too much time on slashdot and Ars.