let's think about this for a second... if you gain them back when you move
back into darkness let's use an example:

You have 1000 max hps and you're full, 1000/1000... you go into some light
area and now you're at 700/700 and you go back into dark, 1000/1000 again...
okay, i'm fine with that and what difference does it really make? maybe even
wizzes would agree it's no big deal... now let's take the same character in a
different situation...

you're now fighting a monster and it's beating you badly... you're down to
100/1000 hps and you flee into a light room where your max drops to 700... now
there are 2 ways for the game to handle this... option a) you lose 300 max and
300 CURRENT hps also... in which case you die... now i'm sure you wouldn't
like this outcome, right?... okay, now other option is your max drops to 700
and you stay at 100... okay, that's fine too... BUT now you move back into a
dark room, and blammo, you're back up to 400/1000 when your 300/300 goes back
on. Now you've benefited from a free 300 hp heal... now this i'm sure you DO
like, but it is obviously not in tune so it wouldn't be done...

Finally the third option is put a flag that determines how many hps you lost
when you went into a light area, and reapplies that number and that number
only ie, you have 850/1000 and you go into light, then you have 700/700 and
when you re-enter you get 150 hps back... but again, depending on what exactly
happens between when you enter the light and when you re-enter the darkness
this will essentially amount to a free heal and won't be done...