... at least, not for me. The bottleneck is something like mental energy. I can only make progress on a (subjectively) difficult topic for a handful of hours (at best) each day before hitting severe diminishing returns, and being forced to "relax" for the rest of the day.
In light of the above, the obvious thing to optimize for the sake of "making progress on difficult things" is the amount of "mental energy available per day", not "waking hours available per day". Sleep seems to serve the function of "restoring mental energy"; but I get diminishing returns on sleeping longer (in a single block) as well; for me, there doesn't seem to be a big difference in "available mental energy" during a day whether I'd slept 6 hours or 9 hours.
So, since I'm getting diminishing returns on both my waking and sleeping hours, the obvious thing to try is to split the day up into multiple "sub days", i.e. some form of polyphasic sleep. Ideally, I'd like to spend a larger total fraction of the day asleep; this contrasts things like "the Uberman's sleep schedule", where the goal is to have as much waking time as possible (which, through this lens, is pure folly).
Have any of you (or anyone you know) had any success with something like this?
You talk about the length of your sleep but have you considered variability of your sleep? For me sleeping always 7h30 seems to greatly increase my productivity in the long run. I sometimes add a few sleep cycle every few week ends but try to stick to a regular sleep schedule instead of thinking in terms of duration.
You might want to try by first waking up always at the same time, regardless of when you fell asleep.