I have a feeling the RAW will suggest that this work just like with any single entity (like a goblin), but affecting a swarm seems like it should not simply be an “all or nothing” consideration. While trying to divvy up the swarm into individual units would be absurd, the description of the swarm already lists a “half-strength” variant. What do people think of an interpretation (or perhaps house rule) that allows you to put half of them to sleep and force them to therefore do half-damage until the other half is awoken? I suppose the complication would be that you would now have to track each individual half’s own hit points, which certainly isn’t ideal. Anyone have any other ideas beyond what the standard might suggest?