DG - Good question. I agree there are many days where I have to force myself onto the bike and even start out by saying I'll just ride for 20 minutes to get myself moving, this is especially true in the winter when it is below freezing and the wind is blowing. I also agree that after 20-30 minutes on the bike my mood usually improves a bunch. Those endorphins are starting to pump through the system. I also have days where I can't wait to get on the bike and blow off some of the stress and "stink" of the day. Today was an interesting ride, I haven't had much sleep the past two nights due to work deliverables and a sick kid. My ride was definitely impacted by this as I didn't have the zip in the legs but I felt strong the whole ride and was in a good mellow groove to just keep rolling along at the same space push the hills a bit and just relax

I've seen a couple of studies that associate cardio activity with a better mental state of mind, this looked at runners and cyclists