Don't let Winter keep you down. Yes there are less hours of daylight every day this time of year...however, there are relatively inexpensive headlights that can easily light your way! Don't let winter spoil your fun, with some half descent lights you can get out and have an awesome ride before or after work.
Definitely take into consideration the fact that you will have less reaction time than usual. But, you will have the opportunity to set out on a new adventure. Whether you just want to blow off some steam, you are training for a big event, or just want to try something different...you will enjoy expanding your experiences by getting out after dark.
I recommend using at least 500 lm for a headlight and another 500 lm light on your handlebar or whatever equipment you are using. Having light from two different angles helps with judging shadows and depth perception. Of course, use common sense: ideally take a friend, or at the very least tell someone where you were going and when you will be back. Just ride within your ability and take it easy the first time out, after some practice you will find that you can more or less go at about the same pace as during the day.
Lately it seems that everyone is out on the trail at JD Wednesday night. If you can't talk any of your buddies in to riding at night, simply ask if you can tag along with another group. If your lucky they might ride at your speed. It helps to have several people so you can get an idea of what turns/obstacles are coming up. The trails are in fantastic shape this time of year, and the people are great too.
Get out there, have fun, be safe, and try something new!