I used to crash about two hours after a workout – that’s about an hour post workout dextrose/protein shake consumption. Basically it is a sugar crash after purposely spiking my insulin after a workout.
I found that I can head that crash off by making sure I eat a solid meal with protein (usually chicken breast)and a low glycemic index cardb like oatmeal or brown rice about an hour after my post workout shake.
Of course it is critical to be sure that you are getting enough sleep in the first place.
I also find that if I have the opportunity, a nap isn’t such a bad thing. If I am tired I figure my body needs it. besides so many good things happen to help recovery during sleep. If you can work it in, why not nap?