In Oblivion you could basiclly cast it as often as you wanted, for no mana cost and it lasted for a fixed duration... actually made it sort've impractical as you had to keep "re-casting" it. One of the first things I'll be doing is making a toggle mod so you use the spell to switch it on/off instead of having to keep casting it.
having it automatic'd be a nuisance if it's like Oblivion, made depth perception and stealth a bit tricky as you couldn't see colour or shadows in night eye mode