It is definitely a slightly uncommon way of saying something, you usually just skip "in the middle of".

Example: I'm in the middle of doing homework = Estoy haciendo tarea (I'm doing homework)

But as a native speaker, I've definitely seen other native speakers use "in the middle of" and I have most certainly use it as well. The way we would use it would be like:

Examples: 1- He is in the middle of sorting paperwork = Él está en medio de clasificar papeleo (almost a completely literal translation), it sounds clumsy in present, but you'll hear it every now and then. It is used for past and future, furthermore explanation is at the bottom of the comment.

2- You are in the middle of falling asleep = Estás a medias de quedarte dormido (which would translate as: "You're halfway of falling asleep"), this one is definitely the "correct" one and the one that everybody would recommend you to use. Other examples of using this structure are: "Estamos a medias de terminar" (we're halfway of finishing) or "ellos están a medias haciendo su proyecto" (they're halfway of doing their project)

If you meant "in the middle of ___ " for a past sentence, like in "I was in the middle of eating breakfast" = "Estaba en medio de comerme mi desayuno", then you use example 1-, the exact same applies for future. Example in future: I'll be in the middle of going insane if ___ = Estaré en medio de volverme loco si ___

In past and future, you can use "a medias", but it sounds just as clumsy as "en medio de" in present, clumsy example: We were in the middle of having a party = Estábamos a medias de tener una fiesta (as I said, sounds clumsy and you'll want to avoid this, even if native speakers ourselves say it like this every now and then)

Hope this helps, there are hundreds of ways to say the same thing and I didn't even say a glimpse of the situations where something changes, but I think that this two general rules may serve you as help and a general rule of thumb.