Lol, I've never gotten up in the middle of the night to play any game, and the minute I do, it'll be time for me to worry about a raging case of OCD.
Working on the first level of Aztec now, and although I hate the random reward style of the merchant, I haven't found it to be all that difficult yet. They really should just make it the same as the Japan merchant though, I don't need a 'game within the game' to keep my attention, and I think it's clearly set up to be a diamond grab IMO.
Didn't like the Viking settlement, but I didn't think it was hard, just tedious to play and look at.
Have done one level of Egypt, and found it somewhat enjoyable, just have to be good with figuring out building placement.
Japan was by far my favorite, and the only one I have fully completed.
The Aztec building looks to be well worth however long it takes, I'll probably circle back to Egypt after I complete that.