Part of it is developmental at this age - the CNing often goes away between 3-6 mths. I'm not sure it is all being overtired or undertired, just the way their little brains are wired at the time, and therefore there is a huge amount of variation between LOs and they are all different.
Both of my LOs were CNers at that age, and both survived on very little sleep. They slept more in total as they got older so something was 'not right' in the early days but there was nothing I could have done to change it at that point. Reflux and wind probably didn't help me
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BUT both ARE low sleep needs. Then both did all the nap transitions early and both have always had longer than average A times.
I remember being on here in the early days and trying trying trying everything to get more sleep, and really they did once they were developmentally able, but did remain low sleep needs.
Oh - and don't underestimate hunger causing short naps - all those GSs in the first 3-4mths don't help!