A book in the afternoon is a recipe for a nap, in my case, about half of the time. I strive to get high quality nightime sleep, and I'm usually successful, getting 8 hours, uninterrupted. But the 8 sometimes is quite "inefficient", taking up to 10 hours. Unfortunately, the waking spells are usually in the wee hours, so getting up for the day isn't an option. And I'm not very good at predicting how long it will take me to get back to sleep, so I just lay there. Half of the time it's 15 minutes and otherwise can go as long as 90. But strangely, I haven't seen an obvious correlation between the previous night and the next day nap proclivity.
One thing I've been much more successful at since retirement is going back to sleep in the morning. In the old days, once awake after 6, that was it. Now, if I haven't got my 8, I can always put the towel over my eyes and get the rest of my zzzz.