One thing I think should be mentioned is that a stable temp is just as important once fermentation has started, if you can only manage to brew at even 22 then it's best IMO to leave it there rather than have it go from 22 to 18 and back up and down all the time as this could stress the yeast, of course, if you can keep it at 18 then thats perfect. Personally, in the middle of summer I can't get my brews down to 18, so 22 is usually the norm, and I still make beautiful brews at those temps, I just leave it on the yeast cake for a bit longer so the yeast can clean up a bit, and I also pitch a bit more yeast to start with.
Sorry if it's already been mentioned and I've missed it, in which case, don't mind me, I'm hungover.