The sleep spell is a bit more severe than naturally falling asleep.
That said, for normal sleep... the
listen skill on the very last line states, "A sleeping character may make Listen checks at a -10 penalty. A successful check awakens the sleeper." So use that with the listen DCs table. If you want to shout to wake someone up as a free action, that would be a pretty easy DC. The table is not comprehensive, it just has examples. But people talking is DC 0 and a battle is DC -10. I would call shouting to be the same volume as a battle, so -10 base DC. The listen DC has modifiers of +1 per 10 ft of distance as well, keep in mind. There's also a +5 DC for "Listener distracted," but I'd say RAI is that is overidden by the more severe penalty of being outright sleeping. RAW, that probably also applies.
So you have someone shout from 30 ft away, DC would be -7 (or -2 if the distracted one counts). You make your listen check at a -10 penalty for being asleep. Passing it means you awaken.
If you want to take an action, you could certainly use a standard action to shake the character awake or make an attack roll (presumably nonlethal) to violently jolt someone awake.
A sleeping character is
unconscious, and thus helpless.
Helpless states:
"A helpless character is paralyzed, held, bound, sleeping, unconscious, or otherwise completely at an opponent’s mercy. A helpless target is treated as having a Dexterity of 0 (-5 modifier). Melee attacks against a helpless target get a +4 bonus (equivalent to attacking a prone target). Ranged attacks gets no special bonus against helpless targets. Rogues can sneak attack helpless targets.
As a full-round action, an enemy can use a melee weapon to deliver a coup de grace to a helpless foe. An enemy can also use a bow or crossbow, provided he is adjacent to the target. The attacker automatically hits and scores a critical hit. (A rogue also gets her sneak attack damage bonus against a helpless foe when delivering a coup de grace.) If the defender survives, he must make a Fortitude save (DC 10 + damage dealt) or die.
Delivering a coup de grace provokes attacks of opportunity.
Creatures that are immune to critical hits do not take critical damage, nor do they need to make Fortitude saves to avoid being killed by a coup de grace."
Hopefully that answers your questions about the effects of being asleep.