I wonder if other basic processes could be at play here like when patients go to the bathroom. If you do this in the morning they may be more likely to not need that for a while while in the evening they may do that immediately. I'm not saying this is the mechanism, just pointing out that there are a lot of timing dependent things in a person's schedule that could be a factor here. It is a great thing to point out though. I hope a lot more research goes into the idea of timing and integrating medication into a schedule most effectively.