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And like someone else's little one, if she's in bed on time she falls asleep no prob, but if she goes up 20 min late she might come down for the next 60 - 90 min, driving me crazy,

I think this is more common that people realize. There is the temptation to assume this means the kid is not sleepy, but it's like with babies, when you miss the nap window (ohhhh, how I HATED that).

Interestingly we don't see this with my higher sleep needs kid (she is mostly OUT a few minutes after her light is out), just the one who is more able to "get by" on less sleep.

I do feel grateful that both of my kids are happy morning people--chipper and alert as soon as they're up (unlike their mother!!) However, they do not sleep in if up very late.