Usage of “when” or “while” in this context

He left the house _ I was still sleeping

Would this sentence be better completed with when or with while?

The choice really hinges on the durative or punctual aspect of the particular verb (or verb construction, like was sleeping) in the adverbial clause. To illustrate the difference:

He left the house when the taxi arrived. (taxi arrived: punctual, point-in-time aspect)

He left the house while I was still sleeping. (I was still sleeping: durative, extended aspect)

There is sometimes a choice, to indicate the intended aspect:

He was in the playground when the children arrived. (They all came essentially at the same time.)

He was in the playground while the children arrived. (They came over a period of time.) (Here, children were arriving would probably be preferable.)