I see no difference in AM vs PM riding until recently. Eight months ago I had ACL replacement surgery, now I need a decent interval of warmup whether it's AM or PM. After 5 or 10 miles of spinning, I can resume riding at my normal pace. I don't worry about a cool down at the end of the ride as much as some good stretches after getting off the bike. Colder weather effects my warmup time more than any other factor.