All of the above advice is spot on. Sometimes I sleep because I am sleepy, and sometimes I sleep because I am tired. Sleep because I am sleepy will be 15-20 min, whereas sleep because I am tired will ideally be in a multiple of 90 min.
During PBP my sleep went:
Monday: woke up at 9:00 am
Monday: started the ride at 9:00 pm
Tuesday: napped for 15-20 min around 8:00 am at Villaines
Tuesday: slept for 3.5 hours (three hours plus 30 min to get to sleep) at 10:00 pm at Loudeac
Wednesday: napped for 30 min at Carhaix (outbound) around 9:00 am
Wednesday: slept for 3.5 hours at 11:00 pm at Loudeac
Thursday: tried to take a micro-nap on the side of the road, sitting still on my bike, but fell asleep until it was too late to keep from falling over (boom!)
Thursday: tossed and turned for 2-3 hours at 11:00 pm at Mortagne before getting up and back on the bike because I was freezing
My sleep plan worked out, except for that last night in Mortagne. I didn't have the right equipment. Instead of a bivy sack, or even a space blanket, I tried to use a liner for my sleeping bag (which was in my drop bag in Loudeac). Both long sleeps in Loudeac were planned. So was the one in Mortagne, and it would have worked out correctly if I had the right thing. By the time I got to Mortagne I was cold, wet, sleepy AND tired.
For the next PBP, and oh yes I will be there, I want to get more sleep at the three planned sleep stops. I really don't care what my finishing time is (I say this now).
FYI, riding straight through Monday night was the first for me. I had no idea how I was going to react.