Technically if you look in the rulebook, the penalty for being attacked WHILE TOTALLY ASLEEP is only -200. Therefore, if you really felt the urge to simulate complete unawareness, go with that.
This is actually a perfect example of what pyroprogamer is saying.
Anima is a game where characters rapidly reach literally superhuman levels. And do superhuman things. Even if they never have 'inhumanity' or the like, a 5th level Weaponmaster (without any Ki powers) can utterly clean house against a bucketload of normal, level 1 characters (who are pretty skilled and deadly themselves).
So for a 'normal' person, a -90 (or the -170 in this situation) would effectively be 'worse than 0' for a modifier, taking them into a negative - their only chance is if they 'bend over to pick up a copper piece' at the right time (ie, score an open roll).
Even a low-level character is pretty hosed.
For a skilled, mid-level person, as noted, it brings them down to around zero - maybe they 'feel something in the air' a scant instant before it hits.
For a high-level person, their trained senses, their honed skills, even a touch of sixth sense born of so many brushes with death, allow them to react nearly instantly with some level of their ability. Perhaps even if it's 'as the attack is touching them', crazy as that might sound.
A really high-level character can even defend somewhat effectively against a low-level character's attack if ambushed while sleeping...
. Shades of the Amber Diceless RPG.
Why? Because they're just that good. :)
But a poor normal (say, your 'average Minstrel', from the Anima book), is rolling his defense with a 20 - 90 (surprised) - 80 (doesn't see it) = -150 modifier...which likely puts him in a WORSE position than if we just said 'okay, he gets 0 defense'.