I chose bedtime which is best for me because it is the only time I can be completely alone. Meditating just before I crawl into bed helps me to sort out any issues I had during the day and put them where they belong. Meditation is different for each of us and if one feels they need it to help them sleep better then that is their way of meditating to find calm and peace. One cannot say "meditation is supposed to be.." any certain way. Meditation is a very personal thing and if you feel you have to follow rules, then what is the purpose of it? The purpose is to bring inner peace and clarity. And if it helps you to sleep, by clarification and enlightment, then that is peace.