I think that riding releases endorphins, and too close to the evening this can cause sleeplessness. I found this earlier in the year but it did pass - and I have learned how late on a week night it is safe for me to ride and still be able to sleep that night.

You have mentioned your 50 mile rides, but what are the other distances you are riding? How often do you ride? Have you considered tweaking your nutrition? While a newcomer myself, I've learned that all of these things do tie together.