Ravenhood,
I have yet to meet someone who can shoot as good or better in the dark, so your senario is common.
The reason why that is...is because our sight picture still plays a huge roll in our accuracy even though an instinctive shooter references it within their subconscious.
A key training tool is to do exactly what you are doing but to do it near a light switch or have someone turn it on inbetween every shot or every 2 or 3 shots so you can see where you are hitting and than try and make adjustments.
What this does is help teach proprioception through visual feedback faster than shooting 3 arrows, walking up to the target and than pulling them and than walking back to your shot distance and starting all over again.
Ray