I think that everyone is different on this point. Many say that the lights of a computer or phone will take down your recall, but I have never had it happen to me. Try it both ways and see what works for you. Also as dark matters said "whatever is easiest for you in the morning". Even if the light hurts you, of you don't want a notepad and pen near your bed it might be easier to just grab your phone for a second, or to just turn a light on for a second. Do what feels comfortable and what works for you. You can write that down in your DJ as well (taken by phone, or by paper) that way you can compare them better.