You mention irregular sleep as a factor here, did you ever try taking a nap every day to see if that helps?

The only time I've had this sort of hyper-productivity feeling, I was working out consistently in the mornings, and always napping around noon, maybe thereby giving a boost in energy levels for "the next stretch" of the day.

I also wish I could reproduce this sensation. For me, I think having clear overarching long term goals, that every routine action built towards helped a lot. Summoning the energy to get started is a lot easier if I have an obvious reason for doing it, and once I get started I usually felt great about it anyways.