I think I may try taking a nap more regularly, but how long should a nap typically last and when is the best time to nap in consideration to the REM cycle? I think I sometimes take too long of naps, but I don't know if it matters that much.

How do I know when I am in an REM cycle? I've heard of some people who use programs designed to predict approximate REM cycle, but I don't know much about those.