I’m grateful for the opportunity to shed some light on this question since the answer is relatively straightforward.
Assuming we are not talking about newborns, the guidelines I’m about to put forward apply to all kids through to elementary school age. All children “should” sleep about 10–12 hours overnight, and for babies and toddlers who still nap, they should nap between 1.5 and 3+ hours a day. If you think about it, nearly every baby and toddler, and even every first grader who is a healthy sleeper, falls into these guidelines. That’s because it’s “normal” for a 7-month-old to nap about 90 to 105 minutes total each day and sleep about 10.5 hours at night. And it’s also normal for some three year olds to nap just shy of 3 hours each day and sleep closer to 12 hours each night. This is because all kids have different sleep needs! If there is a spectrum of “sleep needs,” where some kids need very little and others need a ton, children tend to stick to wherever they are on the spectrum throughout their childhood. If they tend to seem to need less sleep overall to be happy and functional infants when compared to their peers, it’s likely they won’t need a huge amount of sleep as they grow older. Babies who sleep a solid 12 hours a night despite however much they nap during the day may always need a lot of sleep, even once they hit elementary school.
It’s also very typical for babies and toddlers to fluctuate with how long they sleep overnight, depending on their napping habits. For example, an 8-month-old baby who is a strong 11.5-hour-a-night sleeper, and who tends to fall asleep soon after being put to bed at 7 pm, may also start to take longer to fall asleep when put down at bedtime as they inch closer to 16 months (if they still have two naps). In other words, if she typically gets about 3 hours total daytime sleep across two naps from 8 months to 16 months, she may start out with needing 11.5 hours at night and then tend to need less night sleep as she grows older (perhaps as little as 10 hours overnight). But, that same child might then go back to sleeping 12 hours a night when Mom transitions her to one nap. However, it’s safe to assume that until your child is in middle school, they likely need at least 9.5–10 hours of sleep each night at a minimum, regardless of daytime naps — kids need a lot of sleep!
If you find your child is vastly below these averages, don’t panic. If they have not yet learned to fall asleep entirely 100% on their own with no help from you or other sleep props, it’s likely they have a lot of room for improvement – especially for overnight sleep. This is because if your child is reliant on someone or something to fall asleep for naps and bedtime, they won’t be able to fall back to sleep as they wake throughout the night since it’s normal for all humans to wake repeatedly at night when going from one sleep cycle to the next. And, if your child is wholly sleep trained but needs only 9 hours of sleep a night with a 90 minute nap each day, and is otherwise happy, healthy, growing, and thriving, you just may have a very low sleep-needs child. If your baby falls far outside of these ranges, or if you have any other concerns whatsoever, reach out to your pediatrician to check in about your child’s sleep habits.
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