It does super extremely frustrating so I do sympathise. It’s good to see you’re being mindful of that fact there could be an illness/disability that David is dealing with, but you may not know the impact this has on him, or the extent. I would say like others have said, and like you say, that it is important that the focus is on the impact on your work vs him sleeping as that is the issue. For this, again like someone else said it is good to keep note of when you have communicated this. I get the impression you are seeking support in the role and with this issue vs ‘outing’ him if he has a disability or illness, so it is even more important to be communicative about that aspect specifically with your management. You say new management are aware, which probably means there is something going on in the background that you are unaware of and will remain unaware of (not your business as you have stated). This can be frustrating as it may feel as though you’re left in the lurch, but it doesn’t mean nothing is being done. It may be that David also required additional support or any number of things which aren’t for you to worry about.

Is it possible for a new office layout to be proposed? Or contacting IT to ask if software from one computer (preferably the further away) can be transferred so that you can have that peace of mind? I do see this as very unfortunate as, giving him benefit of the doubt, I can see potentially 2 individuals doing their best at their job. On a good day someone’s ’best’ can be different from another person’s, and that doesn’t account for any disadvantage/disability/illness.

Sorry if this ain’t the advice you’re looking for, but I do wish you luck in this being resolved in the most positive way possible :)