I'm assuming you mean "Wizard casts Sleep, mauls any sleeping creatures, then eats them" as a short-cut for the normal way of foraging for food? If that's what you mean, there's plenty of reasons why this wouldn't work out well. And also some for why it would.
The normal act of foraging for sustenance from living creatures depends on a few different things. Generally speaking, the DM would ask you to make a Wisdom(Survival) ability check, which essentially covers all of these:
- You need to find them (a random 20ft radius (40ft diameter) sphere of jungle is unlikely to contain much). Also, line of effect rules (see @gszavae's answer) limit what creatures in the area can actually be affected; being behind a tree trunk from the chosen point would block it.
- You need to get close enough to them (once you pick up a deer's track, you still need to get within 90ft of it. Prey animals are flighty and will run if you're not careful.)
- You need to be fast enough to catch it off-guard (even if you're quietly closing in, casting Sleep requires motion and sound which will alert the critter)
- You need to make sure you can actually eat them (that brightly colored frog might be sleeping, but that doesn't mean it's safe to touch)
- You need to know how to skin and prepare it so as not to waste too much or choke on the bones
Sleep will help you with none of these things. Sleep will help you quickly put a creature to sleep, so it can help with getting sustenance, but it cannot replace looking for food.
So what does Sleep do?
If it were me as a DM and you suggested this, I'd tell that being willing to cast Sleep as part of your attempts to forage for food would totally give you Advantage on that Wisdom(Survival) check, but that's all.
(Unless the adventure revolved so much around getting food that a single check wasn't enough. In that case, you'd be making a bunch of skill checks and choices in your quest for food, but if you got to the point where you're close to prey and in a position to cast Sleep, yeah, that animal would fall asleep)