Everybodies different. Many of the projects I work on are incredibly complex. It takes hours to download intricate understanding of the code and the problem into my brain, and then I have a few hours of very high productivity before the race to exhaustion sets in. When I restart the process the next day, I start without the deep understanding that enabled high productivity and have to be careful not to try to do too much until I once again have that full comprehension of what I am doing.