How to read the water
every age · Updated 2026-08-23
The home screen is a tide. The waterline is your child's sleep pressure — the real, physical drive to sleep — rising while they are awake, draining while they sleep, drawn against their own personal threshold: the level at which they, historically, fall asleep easily. The percentage riding the "now" line says how close the water is to that shore. 100% means "sleep would come fast now". It never means "they must sleep now". Nothing on this screen is a deadline; here is how to read it.
The levels
- 0–30% · rested. The water is low. A sleep attempt now will almost certainly fail — there is nothing to release. This is not a problem to fix; it is a child being awake.
- 30–85% · building. The normal play zone. Nothing to do, nothing to watch.
- 85–97% · winding down. Calm play, start the routine; sleep is possible with help.
- 97–107% · the wave. The easiest fall-asleep, typically within about 5–15 minutes of trying. This band is deliberately wide — it is a window, not a point.
- Past ~107% · the wave passed. Second-wind territory: paradoxically harder to settle. Fully recoverable — carry gently, and there is always a next wave. Aim a little earlier for it.
Three things the number is not
It is not a battery. Pressure builds fast and then decelerates, so 50% is not halfway in time — for my own child it was roughly a third of the way to the wave. And a nap doesn't drain the water to zero: a fair afternoon nap from a high line lands around 25–30%, so afternoons start with a head start. Only a full night empties it.
It is not a countdown. The most honest translation of the % is how long would falling asleep take if we tried now — at low water even a perfectly executed nap attempt means a long latency or refusal; on the wave it means minutes. (This latency link is measured, not folklore: in 2–5-year-olds, sleep latency runs about 24 minutes at low pressure and saturates near 13 after ~7 h awake.)
It is not a verdict. Your child's cues outrank it. The water tells you what the biology has had time to build; your eyes tell you what today did with it.
Past, future, and the blur
The line is sharp where the day already happened and softens as it runs ahead, because the uncertainty is about when the next transition lands, not what the physics does after it. A running nap keeps a sharp shape; it is the wake at its end that smears. If the future ever looks hazy to you, that is the app being honest, not broken. The compact strip below the water shows the whole day in the same terms — solid for sleep that happened, hollow for sleep merely expected.
One distinction worth knowing, in the details pane: prescribe is where the raw physics says the water reaches the shore — where to steer. Predict is where your family's actual habits say sleep will really start — what is likely. The gap between them is information, not error: it is how far your logged routine has drifted from the biology.
What to actually do with it
Glance, see where the water stands, close the app. That's the intended use — five seconds. Believe the window, not the minute: the best prediction error I can honestly achieve is 17–21 minutes (the full scorecard, losses included), and the design assumes exactly that softness. If the water and your child disagree, the child is right — and logging what actually happened is how the tide gets less wrong tomorrow.
Where this comes from
- Salome Kurth et al., "Development of nap neurophysiology: preliminary insights into sleep regulation in early childhood", Journal of Sleep Research (2016). Sleep latency vs. time awake in 2–5-year-olds: ~24 min at low pressure, saturating to ~13 min after ~7 h awake.
- My own dataset: 671 days, 2,181 hand-logged sleep sessions of one child, month 1 to month 22, used to fit and validate the engine. The honest scorecard, losses included, is in the notebook.