Heya, me again
To basically sum it up, night sleep begins when your lo says it begins, which is biorhythmically different from day sleep. Technically, the night biorhythm is influenced by the dark/light cycle of the day, and is based on when your lo's last nap ended, how tired they are, etc. So it usually kicks in anytime after 5 pm (when it starts to get dark). So putting your lo down anytime after 5 pm spawns the possibility of sleeping through the night, because melatonin kicks in when it's dark (among other things).
But then, the biorhythm is also a genetic thing that doesn't really take shape until after about 4 months, when night and day sleep finally finish development. After that happens, a pattern usually develops, and you can usually use their morning wake up time (assuming it's around the same time every day) as a target bedtime at night, and fit naps in between. Some take a short nap after 5 pm and go to bed at 8. That's fine as long as they are getting 10-12 hours of sleep before they are "up for the day" (meaning no "A" time aside from a possible feeding, then immediately back to bed).
If he happens to wake at say, 6 am, and falls back to sleep, this still counts as night sleep. Technically speaking (although the research jury is still out on this one), the 1st nap taken in the am is actually a continuation of night sleep, even though it happens after some "A" time. The biorhythm readout for that sleep period is about the same as it is in the early morning hours...filled with lots of REM sleep, whereas the afternoon nap is filled with deeper sleep, and has a different rhythm.
Finally, bedtime/wake time is habitual. So can nap time. The body adjusts as something becomes usual...so if you put your lo to bed at around 8 pm every night, and they get up at 8 am every morning (although that time is really biorhythmical and is hard to change), their biorhythm will adjust to it in time.
Biorhythmically, the best times for naps are around 9 am and 1 pm, give or take an hour on each side. But you need to really base your nap times on how long your lo has been up; usually the best chance for a long nap (and good, quality sleep) falls within those time slots though.
Even more technical is that the quality of sleep counts more than the amount of sleep. How do you know if he's getting enough sleep? Use his behavior. If he's his normal self, he's getting enough sleep. If he's cranky, not interested in learning, dependant on you to keep him busy when he's normally not, etc. he's probably not getting quality sleep, even if he's just taken a 1.5 hour nap.
I'm not sure if much of this is going to answer your questions (as much as confuse you lol), but I hope it helps a little
*HUGS*
Sonya =P
