DD has always been bit low on sleep needs compared to what I read for lots here. Before daylight savings she did solid 11 at night and 2hr nap.
Since she hit 18 months and since daylight savings she does 10 or 10.5 at night and 2hr nap. It adds up and she seems mostly fine but definitely overtired some days. She talks herself to sleep at night up to 40 mins and that is the sleep loss.
Putting down earlier hasn't helped and I fear later since she's early riser already.
Re: Less sleep at 20 months
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DD hasn't slept 13hrs a day in ages. When she dropped her second nap around 18mo, she would get, at best, a 1.5hr nap and 10.5hrs at night. Usually it was more like 9.5-10hrs at night. Now that she's dropped her last nap, she gets, at best, 11.5hrs a day (7:30pm to 6:30 or 7am).
Going by behavior is best, and if your LO seems still tired, trying for more sleep might be good, but it may or may not work.
Good luck,and know you're not alone.
Our son is the same age and this has been going on with him for the past month as well. About an hour total sleep loss in 24 hours. His nights have gotten back to 11 or 11.5, but naps are sometimes only an hour. Also, he's gone from taking about 15 mins to fall asleep to now it takes 30 mins at least! He's not crying, but just rolling around and looking around, but quiet. He is starting to really talk a lot more and repeat everthing we say, so I think that's what it is. I agree, if behavior is still pretty good, it's probably ok. Also, it could be a phase.