Why is taking a nap before bedtime bad for REM sleep?

I take a nap after getting home because I'm just too exhausted to focus on my work. So, I decide to take a nap, and I wake anywhere from around 11PM to 2 or 3AM, and I go back to sleep again from anywhere around 3-5AM and wake at 6AM. I heard that doing this disrupts the REM cycle. If so, how? And is it really that bad? What are the long term side effects?