Oh dear. At some point the wizard is going to have a second Sleep spell. At which point they own another encounter. And more spells, and more powerful spells. Which probably means more encounters being owned by the wizard.
Let us hope there are lots of doorways that need to be blocked against a horde of weak attackers in D&DN.
I've been playing D&D since the late 70s from Basic, Advanced, etc, through 3E and 4E and have never found Sleep to be this super "I win" button as either a player or a DM. In 3.5E, it only affects up to 4HD of creatures, requires a full round to cast and requires V, S and M components. It also is a Will negates and does not affect undead or constructs. So, if the wizard memorized sleep as one of his first level spells for that day and you encounter skeletons and zombies, you're out of luck. If you cast Sleep at 4 kobolds, chances are that at least one of them makes a save - if they don't detect the wizard first while he's chanting his magic words and waving his arms around for a full round.