Big productivity boost if you're setting your own schedule or you're often being the night owl --
Once you start to feel your mind wind down and the fog set in, sleep ASAP!
It sounds simple, but the execution is life-changing. I used to waste about the last two hours of the day in a daze on many days where I wasn't totally spent at the end of the day. Now I've got it down to usually recognizing between 40 and 90 minutes that my mind isn't going to clear, and crashing out.
There's no sense trying to push through it if legitimate exhaustion is there... the quality of thinking will be low. If you must stay up (deadline, etc) grab a cold shower or go running or do pushups. But if you don't have to -- grab some sleep. The you-of-tomorrow will be grateful you did.