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.
- Each attack will have advantage.
- Each attack that hits is an automatic critical hit (and will probably kill the target).
- A creature waking should almost certainly be considered surprised, and not be able to act on its first turn.
2) Alternatively if you're intending to sneak around and do things without waking them up or starting a fight, then again:
- Do the sneaking and stand ready to cast sleep if any of the creatures do happen to wake up.