My DS's doctor said as long as we let him nap whenever he is tired, he will naturally get all the sleep he needs. I actually notice that he sleeps much better when he falls asleep later (11-12pm). I'm not surprised because *I* was the same way as a kid, still am -- I do better going to bed at 10-12pm & waking up around 9 or 10am. Some people's biological clocks are just shifted a bit!!! Plus when you factor in daylight savings time & regional differences in sunset/sunrise etc. I don't see how, scientifically, 8pm or earlier would be "optimal" (unless like you said, they're assuming early wake up).
Co-sleeping is really wonderful when your child actually SLEEPS!!