I would be willing to bet, they're not afraid of the dark per se, but rather the sudden dark. If you really want to alleviate that reaction you could invest in a ramping timer. These things are used for aquariums mostly, but it's the same idea, it mimics the natural light cycle.
This is just the first one I ran across, I'm sure there are probably cheaper alternatives although like anything else you get what you pay for.
Ramp Timer Pro (Amazon)
This is just the first one I ran across, I'm sure there are probably cheaper alternatives although like anything else you get what you pay for.
Ramp Timer Pro (Amazon)