Its up to the GM
The Sleep spell does not clearly specify what happens.
Casting Number: 6
Casting Time: Half action
Ingredient: A piece of down (+1)
Description: Your touch causes one opponent to fall into a slumber
for 1d10 rounds unless a successful Will Power Test is made.
Sleeping characters are considered to be helpless. Sleep is a
The only relevant information here is that the target is asleep (fall into a slumber) and helpless. Slumber is not really a condition well defined by the rules, and Helpless (pg 133) says:
Helpless: The character cannot defend himself due to
wounds, incapacitation, or some other circumstance. Attacks
against the character automatically hit and inflict an extra
But how deep are they sleeping? I know I sleep like a rock sometimes, and probably even a sword stab wouldn't wake me up, but not everybody is like that, some people are really light sleepers.
I know that the spell Winter's Long Slumber (Realms of Sorcery, pg 141) puts the (willing) target to sleep for several months and the spell clearly states that normal damage has no effect on them, they must wake up on their own will. On the same book, the Lucidity Tonic also will put you to sleep without interruptions for several hours.
If we check the definition of the word Slumber, we see that it is most commonly associated with a light sleep (to my surprise, I always thought it was the opposite), so I personally would say that if the target takes any damage, they should wake up.
Keep in mind that helpless targets take an additional 1d10 damage, but that is more aimed towards being attacked by an enemy trying to kill you, not an ally trying to wake you up. So it's better to ignore that rule when the objective is not to kill the slumbering target.