How to Calculate Your Baby's Sleep Need and Build a Routine Around It
Once you understand that every baby has their own individual 24-hour sleep need, the natural next question is: how do I actually find out what my baby's number is, and what do I do with it once I have it? Here's exactly how.
Step 1: Track Total Sleep for At Least 5 Days
Start by logging every single sleep your baby has across a 24-hour period, both naps and night sleep, for a minimum of five days.
For each sleep, note:
The time they actually fell asleep (not just when you put them down)
The time they woke up (not just when you got them up)
Being precise here matters far more than it might seem. The gap between "time in the cot" and "time actually asleep" can be significant, especially if your baby takes a while to settle, and it'll skew your number if you're not careful.
Step 2: Add Up the Total for Each 24-Hour Period
For each of your five (or more) days, add together the total time spent asleep: every nap plus the full night sleep.
Step 3: Average It Across the Days You Tracked
Once you have a daily total for each day, add those totals together and divide by the number of days you tracked. This gives you your baby's average 24-hour sleep need.
A few tracking tips that make a real difference:
Choose representative days. Track days that reflect your baby's normal routine, rather than days disrupted by illness, travel, or a growth spurt. At the same time, don't cherry-pick only the smoothest days either; you want an honest average, not a best-case scenario.
Track for longer if the numbers vary a lot. If your five days throw up quite different totals, it's worth tracking for a few extra days.
Be honest about wake-ups. If your baby wakes overnight and it takes them a while to resettle, that time awake doesn't count as sleep, even if they're lying quietly in the cot.
Don't adjust your usual routine while tracking. The goal is to capture what your baby's sleep currently looks like, not what you think it should look like.
Step 4: Decide How That Total Splits Between Night and Naps
Knowing the total is only half the picture. Two babies with exactly the same 24-hour number could still need very different routines, depending on how that sleep needs to be divided between night and day.
As a general rule, the majority of your baby's sleep should be concentrated overnight, with naps making up the smaller remaining portion. Night sleep typically falls somewhere between 10 and 12 hours, with the remainder of your baby's total sleep need split across however many naps they're having that day. That split doesn't need to be even; it's really up to you how you divide it, and over time you may well find your baby naturally shows a preference for one nap being longer than the others.
How the Split Shifts With Age
As your baby gets older, this balance shifts too. Younger babies tend to need several shorter naps to make up their daytime portion, while older babies need far less total nap time, consolidated into just one or two longer naps, with night sleep taking up an even greater share of the 24-hour total.
This is one of the reasons a routine that worked well a few months ago can suddenly stop working: your baby's total sleep need may have changed, and the ideal split between night and day will have shifted with it.
Building the Routine
Once you know roughly how your baby's total should divide between night and day, you can shape naps and bedtime around that split, rather than around a generic timetable. In practice, this means:
Deciding how many naps your baby currently needs, based on their age and total daytime sleep requirement
Spacing those naps so each one falls when genuine sleep pressure has built up
Protecting enough of a gap before bedtime to allow sleep pressure to build for the night
If you'd like to understand more about what's actually happening in your baby's body to create that sleep pressure, I've written about the science behind it here: What Is Adenosine, and Why Does It Matter for Baby Sleep?
And if you're working towards sleep training, getting the total and the split right isn't optional groundwork, it's essential. I explain exactly why in Why You Must Calculate Sleep Need Before Sleep Training.
The Takeaway
Five days of careful tracking, followed by working out the right night-to-day split, is a small investment for a genuinely useful result: a personalised routine built around what your baby's body actually needs, rather than what a generic chart assumes.