One model, three faces
All three tools read from the same table of age bands, which is why they never contradict each other. Each band carries an awake-time range, a nap count range, a total daytime sleep range, an overnight span, a 24-hour total, and — importantly — the name of the body that published that total. Nothing in that table is our opinion, and the parts that are weakly sourced are labelled as weakly sourced rather than quietly smoothed over.
Four bands, as a sample of what the tools are working from:
Why the schedule builder works backward
This is the design decision that makes the biggest practical difference, so it is worth two paragraphs. Most schedule calculators work forward: take the morning wake time, add a wake window, place a nap, add the nap length, add the next window, and so on until the naps run out. Whatever time the last window ends is declared bedtime.
The trouble is that errors accumulate in one direction. A slightly long nap and a slightly generous window, repeated three times, is an hour of drift by the evening, and the tool reports a 9:15pm bedtime with total confidence. Ours solves in the other direction: it starts from how much sleep the age band expects across the whole 24 hours, reserves the overnight span, and fits the naps and awake stretches into the daylight that remains. If they do not fit, it tells you so and drops a nap rather than silently pushing bedtime into the night. If the arithmetic wants a bedtime before five in the afternoon — which a no-nap day with an early start genuinely produces — it refuses, holds the floor at 5pm, and explains why instead of printing absurd advice.
It also distributes the awake stretches unevenly, shortest first and longest before bed, because that is the shape published sample days consistently show. A single repeated number produces a morning nap that fights you and an evening that unravels.
The number we refuse to show you
There is an obvious product on this site that we are not going to build: a percentile. “Your baby sleeps better than 68% of babies her age” is compelling, shareable, and completely unavailable to us, because it would require a dataset of real babies that we do not have. Plenty of tools display that number anyway. Where they get it from is a question worth asking.
What the comparison card does instead is narrower and true: it says where a number sits inside a published range and names the panel that published it. That is less satisfying and considerably more useful, because the published ranges are wide for a reason. The AASM range for a six-month-old is twelve to sixteen hours — a four-hour spread, published that wide because healthy babies genuinely vary that much. A baby at the bottom and a baby at the top are both fine, and no ranking would tell you anything except how to feel worse.
When to trust your baby over the tool
Always, in the specific sense that a live baby is better evidence than an average. The tools are there to tell you where to start looking, not to overrule what you can see. Some concrete cases where the output is wrong and your baby is right:
- She is plainly finished before the window closes. Heavy eyes, staring, a face turned away — go now. A nap started at the right moment for her beats one started at the right moment for the median baby of her age.
- She is cheerful and playing well past the window. Some babies sit above their band. Forcing a nap into a baby with no sleep pressure produces twenty-five minutes of chatting in the cot and a shorter nap, not a longer one.
- Today is not a normal day. Teething, a cold, a vaccination, travel, a new motor skill she has just learned and now wants to practise at 2am. All of these break the model, and none of them are scheduling problems.
- The plan requires you to wake a sleeping baby. The tool will occasionally imply capping a nap to protect bedtime. That is sometimes right and often not worth it. Use judgement.
- Something feels wrong rather than merely inconvenient. Feeding changes, a mood that is deteriorating week over week, breathing that worries you. That is a pediatrician conversation and no calculator belongs anywhere near it.
What these tools cannot see
A short and honest list. They do not know your baby’s temperament, which is probably the single largest source of variation between two babies of identical age. They do not know whether she was born early, and adjusted age matters considerably in the first year. They do not know about feeding, reflux, or illness. They do not know that she naps brilliantly in the pram and terribly in the cot. And they cannot distinguish a genuine developmental change from a rough fortnight, which is the hardest judgement in the whole first year and the reason the transition checker is deliberately hedged.
They also do not touch safe sleep at all. Where and how your baby sleeps — surface, position, what is in the cot — is not something a calculator should have an opinion about. Go to NICHD’s Safe to Sleep and the AAP for that, and to your pediatrician for anything specific to your baby.
