Question: Why do some children have long naps, and other children have short naps?
Answer: It partially depends on the bedtime
When the bedtime is early, children wake up well rested. Because there is a genetic effect on the duration of naps, some children take long naps and other children take short naps. There is also a separate genetic effect on the duration of night sleep. In this situation, there is no association between the duration of night sleep and the duration of day sleep (naps). If your firstborn child took long naps, and you got used to this free time during the day, you might be frustrated if your second child takes short naps. You cannot make this second child take long naps.
When the bedtime is too late, children do not wake up well rested, even with a late wake-up time, because they are not sleeping in synchrony with their circadian sleep rhythms (Blog Posts 62 and 112). Also, usually, a later wake-up time, does not fully compensate for a later bedtime so that the duration of night sleep is shorter. Therefore, throughout the day, there is increased sleepiness that causes long naps. Here too, usually, the longer naps do not fully compensate for the shorter night sleep, so the duration of total (24-hour) sleep is shorter. In this situation, there is an association between the duration of night sleep and the duration of day sleep (naps). That is, the shorter the night sleep, the longer the naps. The late bedtime is masking the genetic effect on the duration of naps. Also, in this situation, because sleep is not aligned with circadian sleep rhythms, short sleep duration, or both, there are many possible adverse outcomes for the child. Some researchers have mistakenly focused on the long nap(s) as the problem instead of correctly focusing on the parent-caused too late bedtime.
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