I have not studied mana regeneration in CODS (at least, not nearly as much as I have in Bane), but in Bane, each character slot regenerated once per hour (60 moves/turns/clicks). I.e., if you had a divine regeneration rate of 4, then once per hour, you would regenerate 1-4 points of divine mana. (I don't recall, atm, if "0" was also a possible regeneration result.)However, each character slot (again, this is Bane I'm discussing) does not regenerate at the same time!
For example, slot 1 regenerates at 0:05; slot 2 at 0:15; slot 3 at 0:25; etc. Thus, with obsessive-compulsive micro-managing, your main spell caster can be shifted through the different slots just as they reach their regeneration point, with the result that one character regenerates mana six times faster than normal. (At the price of other characters not regenerating at all; or two characters can regenerate three times as fast; etc.)
Of course, you can't do this sort of micro-managing in your sleep, or while your characters are sleeping. ;)
Anyway, the real bottom line: resting is the worst possible way to regenerate mana in the game.