I try to take a nap (under 30min) after I train but it is easy for me because I work from home. I also walk a lot which helps me sleep better at night.
When I first started to nap I couldn't relax enough to sleep. I just ended up tossing and turning. But if you do it at the same time for a few weeks your body begins to adjust. If you sleep too long it will likely work against you because you won't be able to slep at night, so keep your naps short...
For me it is just a way to wind down, to destress and regain my focus...