Catching crappie at night can be very productive. You have to do some learning on the water you fish. Crappie arent EVERYWHERE but the more times you go and look at the sonar/maps studying your lake the better you'll become about locating and catching crappie. I'd say anytime is a good time to fish. I prefer water temps in the 50's atleast, seems the fish are more active once it gets that warm. As spring starts getting here the fishing picks up. About the only time there is a little lull in the night fishing action is when the fish are against the banks spawning. That generally doesnt last long and it's back to night fishing until it turns hot summer when I start loosing interested due to all the heat and tired of the bugs by then. I start back in the latter part of the fall/early winter months.
Health nuts are going to feel stupid someday, lying in hospitals dying of nothing.