Depends how you manage it. When I do this, I go to sleep mid-afternoon the next day, and sleep for 12+ hours. When I wake up after that (early morning), I feel

Depends how you manage it. When I do this, I go to sleep mid-afternoon the next day, and sleep for 12+ hours. When I wake up after that (early morning), I feel absolutely fantastic. The end result is probably only about 20-25 hours work in two days - i.e. not much more than normal working days. The advantage isn't in the total hours, it's in having the same problems rattling around in your head for a long time.