If an enemy is put to sleep using the sleep spell and then attacked by a rogue, does the rogue get the full attack bonus (auto crit, sneak attack, etc.) even if other enemies are fully aware of the combat before the enemy wakes up? This seems to be a really powerful combination especially at lower levels.
Attacks made within five feet of the target get sneak attack and critical hit.
- An unconscious creature is incapacitated, can't move or speak, and is unaware of its surroundings.
- The creature drops whatever it's holding and falls prone.
- The creature automatically fails Strength and Dexterity saving throws.
- Attack rolls against the creature have advantage.
- Any attack that hits the creature is a critical hit if the attacker is within 5 feet of the creature.
From the fourth bullet, you would have advantage (as long as you don't also have disadvantage for some reason), which is good enough to trigger sneak attack damage:
Once per turn, you can deal an extra 1d6 damage to one creature you hit with an attack if you have advantage on the attack roll.
From the fifth bullet, if within five feet of the target, your hit would be a critical hit, assuming it hits.
Note, ranged attacks are not subject to disadvantage at close range in this situation. The rules for ranged attacks say:
When you make a ranged attack with a weapon, a spell, or some other means, you have disadvantage on the attack roll if you are within 5 feet of a hostile creature who can see you and who isn't incapacitated.
Since our target is incapacitated, we don’t have disadvantage for being too close with our ranged attacks
Attacking from more than 5 feet away is more complicated.
The second bullet point of unconscious tell us the target is now prone, and the prone condition says:
An attack roll against the creature has advantage if the attacker is within 5 feet of the creature. Otherwise, the attack roll has disadvantage.
This makes things more complicated for ranged attack users - the disadvantage cancels advantage, so we wouldn’t get sneak attack unless we can satisfy the other point of sneak attack:
You don’t need advantage on the attack roll if another enemy of the target is within 5 feet of it, that enemy isn’t incapacitated, and you don’t have disadvantage on the attack roll.
If you can satisfy this, you still get sneak attack, but you would not get an automatic critical hit.
The 'Full Attack Bonus'
Thomas Markov's thorough answer explains why you do get "the full attack bonus"
Attacking an unconscious creature gives you advantage
If there is not disadvantage, you then get sneak attack
Attacking an unconscious creature within 5 feet gives you automatic criticals
One thing that his answer does not make explicit, is that you not only get a critical hit extra die on your weapon damage, but on your sneak attack damage as well (and see How does extra damage work for critical hits?)
So yes, the sleep / rogue combination gets "the full attack bonus" even if other opponents are aware of the rogue, and this can add up to a lot of damage dice.
A really powerful combination?
Is this a really powerful combination, especially at lower levels?
Well, that depends on what the opportunity cost is for using your rogue's attack on sleeping opponents.
If all your opponents are asleep, you hold the field anyway, and can kill them (or tie them up) at your leisure.
But if only some of them are asleep, and others aren't, do you really want your rogue to be the one dispatching the sleeping ones? Consider that:
Sleep puts creatures down in inverse order of hp, so the ones sleeping are likely to have fewer hit points than the ones still up
Your rogue has high burst damage
Any opponent still awake can awaken sleeping allies by spending an action to shake them
So, rather than killing sleeping opponents, it might be a better use of your rogue's 'particular set of skills' to be attacking or constraining the movement of the higher hp opponents still awake, before they rouse their colleagues.
You might do better to have some of the lower DPT team members assigned to slaying the sleeping opponents, on whom the rogue's comparatively high damage would likely be wasted.
In fact, for dispatching sleeping opponents at low levels, I would recommend sending the monk, not the rogue. High movement rate and many attacks of low damage each should slay the most sleeping opponents per round.