Does one notice being affected by the sleep spell?
The reason I ask is because I want to ensure someone who is already asleep via natural means will not wake up for the next 60 seconds. To do so, I will cast sleep on the already sleeping target(s).
Might the target be awoken by this action if the sleep spell fails to take effect?
This spell sends creatures into a magical slumber. Roll 5d8; the total is how many hit points of creatures this spell can affect. Creatures within 20 feet of a point you choose within range are affected in ascending order of their current hit points (ignoring unconscious creatures). Starting with the creature that has the lowest current hit points, each creature affected by this spell falls unconscious until the spell ends, the sleeper takes damage, or someone uses an action to shake or slap the sleeper awake.
A sleeping creature is considered unconscious (as the spell's effect suggests). Therefore, your sleeping target is not affected by the spell.
The spell doesn't affect already sleeping creatures at all. Strictly speaking, the spell doesn't target creatures, it targets an area; and affects or ignores creatures according to their current hitpoints and whether they're already unconscious.
Creatures ... are affected in ascending order of their current hit points (ignoring unconscious creatures).
... each creature affected by this spell falls unconscious until ...
As a result, you won't wake them up. You also won't be able to extend the duration of their sleep.
Note however: When they do wake up, they will probably be prone, and you should have little trouble putting them back to sleep if desired.
Some of the effects of being unconscious:
- The creature drops whatever it's holding and falls prone.
- Attack rolls against the creature have advantage.
- Any attack that hits the creature is a critical hit if the attacker if within 5 feet.
Given the above, the creature would be quite lucky to wake up alive. ;)
In comments and edits to your question: you clarify that you're asking about creatures that are sleeping naturally.
1) Presumably you're trying to ensure they are affected by magical sleep which would prevent them from waking up in the commotion of battle.
Anything you do close to the creatures may risk disturbing them (possibly need a stealth check to avoid waking any of them - either at a low DC or with advantage). Note that Sleep has a 90 foot range so you can easily keep your distance.
However, as already stated, the sleep spell has no effect on sleeping creatures (sleep is a form of unconsciousness); it will not convert natural sleep into magical sleep, nor will it wake any of the creatures up.
So your best course of action would probably be to have the stealthiest character in the party sneak up and slit their throats. While the magic user stands ready to cast Sleep if any do wake up.
Note:
2) Alternatively if you're intending to sneak around and do things without waking them up or starting a fight, then again:
On the one hand,
Unless a spell has a perceptible effect, a creature might not know it was targeted by a spell at all. An effect like crackling lightning is obvious, but a more subtle effect, such as an attempt to read a creature’s thoughts, typically goes unnoticed, unless a spell says otherwise.
So the Sleep spell itself won't wake up a creature who it fails to affect, since it didn't have a perceptible effect. Or any effect at all, for that matter!
On the other hand, Sleep has a verbal component.
Most spells require the chanting of mystic words.
If someone was chanting mystic words at me, I'd like to think that I'd wake up. So you might wake the creature up by casting the spell, even though the spell itself wouldn't wake them up.
