A curious phenomenon...

When it comes to falling asleep at night, I find it pretty much impossible to do so while lying flat on my back; I must turn onto a side to drift off into the arms of Morpheus. As a result, until fairly recently, falling asleep at night has been quite difficult, often accomplished by half-turning on my side, as lying completely on one side or the other did funny things to the arrangement of bones healing in my chest.

Staying on one side for several minutes seemed to "reconfigure" my chest—and probably not for the better—as returning to the flat-on-my-back position was accompanied by a "re-reconfiguration" as everything more or less got back to normal.

However, yesterday I noticed that any time I take a nap in the afternoon, I start out flat on my back and drift off with no problem at all. And when I wake up, I'm still flat on my back.

I wonder why that is?