How SleepSense Works

A plain-English look at what the algorithm does and why it works

The formula

SleepSense looks at three things to figure out when your baby needs sleep:

Next nap = How long baby can stay awake at this age + time-of-day adjustment + what we've learned from your baby

1. Age-Based Wake Windows

How long babies at each age can comfortably stay awake before they need sleep. These ranges come from pediatric sleep studies.

2. Time-of-Day Adjustment

Babies (and adults!) naturally get sleepier at certain times of day — like mid-morning and early afternoon. We factor that in.

3. Personal Pattern Detection

After 5+ days of tracking, nappi starts to learn your baby's own rhythm and blends it with the general guidelines.

Complete SleepSense reference table

Every number the algorithm uses, all in one place. This is what SleepSense starts with before it learns your baby's personal patterns.

Age	Wake Window	Naps	Total Sleep	Night	Day Naps	Nap Length	Bedtime	Min Catnap
0-2 weeks	45 min	6	16h	8h	8h	30-45 min	9-11 PM	30 min
2-4 weeks	1h 20m	5	15.5h	8h	7.5h	30-45 min	9-11 PM	30 min
4-6 weeks	1h 35m	4	15.5h	8.5h	7h	30-45 min	9-11 PM	30 min
6-8 weeks	1h 45m	4	15h	8.5h	6.5h	30-60 min	8-10 PM	30 min
8-10 weeks	1h 55m	4	15h	9.5h	5.5h	30-60 min	8-10 PM	30 min
10-12 weeks	2h 5m	3	15h	9.5h	5.5h	30-60 min	8-10 PM	30 min
3-4 months	1h 55m	3	14.5h	10h	4.5h	45-60 min	7:30-9 PM	35 min
4-5 months	1h 55m	3	14h	10h	4h	45-60 min	7-8 PM	35 min
5-6 months	2h 10m	3	14h	10.5h	3.5h	45-90 min	7-8 PM	35 min
6-7 months	2h 25m	3	13.5h	10.5h	3h	60-90 min	6:30-7:30 PM	40 min
7-8 months	2h 40m	2-3	13.5h	10.5h	3h	60-90 min	6:30-7:30 PM	40 min
8-9 months	2h 55m	2	13h	10.5h	2.5h	60-120 min	6:30-7:30 PM	40 min
9-10 months	3h 10m	2	13h	10.5h	2.5h	60-120 min	6:30-7:30 PM	40 min
10-12 months	3h 30m	2	12.5h	10h	2.5h	60-120 min	6:30-7:30 PM	40 min
12-14 months	3h 50m	2	12.5h	10h	2.5h	90-150 min	7-8 PM	45 min
14-18 months	4h 40m	1-2	12h	10h	2h	90-150 min	7-8 PM	45 min
18-24 months	5h 40m	1	12h	10.5h	1.5h	90-150 min	7-8 PM	45 min
24+ months	6h 10m	1	11.5h	10.5h	1h	90-150 min	7-8 PM	45 min

Wake windows and sleep totals come from different studies and won't always add to exactly 24 hours. That's normal — real babies don't follow a formula. SleepSense uses wake windows as the primary driver and adjusts nap durations based on your baby's actual patterns.

How tiredness builds

Your baby's need for sleep doesn't build at a steady rate. It starts slow after waking, ramps up in the middle of awake time, then levels off:

Low: Baby just woke up or hasn't been awake very long. Wide awake and happy — not ready for sleep yet.
Building: Getting closer to nap time. You might start seeing early sleepy cues like yawning or eye-rubbing.
Just right: This is the sweet spot. Baby is tired enough to fall asleep easily, but not so tired they're fussy.
Overtired: Stayed awake too long. Stress hormones kick in, which can actually make it harder to fall asleep.

Time-of-day patterns

Babies naturally get sleepier at certain times. SleepSense adjusts its predictions based on these patterns:

Mid-morning (9-11 AM): A natural dip in energy — many babies are ready for their first nap around here (-10% on wake window)
Early afternoon (1-3 PM): The sleepiest part of the day for most babies (-15% on wake window)
Late afternoon (5-7 PM): A burst of energy — the "second wind" that makes late naps tricky (+10% on wake window)
Evening (7-9 PM): Energy dips again as the body winds down for the night (-10% on wake window)

Wake windows and nap durations by age

This is the baseline data SleepSense starts with. Naps start short (about 45 minutes) for newborns and get longer as babies learn to string sleep cycles together:

See the full wake windows chart with calculator →

Age-Appropriate Bedtimes

Bedtime naturally shifts earlier as babies grow. Before 3 months, late bedtimes (9-11 PM) are completely normal — your baby's internal clock is still developing. By 4-6 months, most families settle into a 7-8 PM bedtime.

See the full bedtime chart with calculator →

Bedtime-Optimized Catnaps

When the last nap would push bedtime too late, SleepSense caps it at one sleep cycle. Just enough sleep to take the edge off so your baby isn't overtired at bedtime, but short enough to keep bedtime on track.

Here's what happens behind the scenes: SleepSense works backwards from your bedtime setting, figures out when the last nap needs to end, and shortens it to one sleep cycle if a full nap would push bedtime too late. It won't cut the awake time before the catnap too short, though — baby still needs to be tired enough to actually fall asleep.

Catching up on lost sleep

SleepSense looks at how much your baby slept over the last 3 days and compares it to what's typical for their age. If they're running on less sleep than usual, wake windows get shortened by up to 10% — a tired baby needs rest sooner. If they had extra sleep, windows stretch a little. This keeps predictions accurate after a rough night or an unusually long nap day.

Sleep Regressions

Around 4, 8-10, 12, 18, and 24 months, babies go through big developmental leaps that can throw their sleep off. SleepSense knows about these phases and widens its prediction window to give you more flexibility. You'll see a heads-up when your baby is in one, so you know it's normal (and temporary).

Check if your baby is in a regression →

Confidence Scoring

Every prediction comes with a confidence level so you know how much to trust it:

Days 1-3: Guidelines mode — age-based suggestions while SleepSense gets to know your baby

50-65% — Starting to see patterns (3-5 days of tracking)

65-80% — Solid patterns forming (5-10 days)

80%+ — Highly personalized (10+ days of consistent data)

SleepSense needs at least 3 days of sleep data before it starts making predictions. Until then, it shows age-based guidelines. Once predictions kick in:

Confidence = 50% (general sleep science) + up to 50% (how well it knows your baby)

The research behind it

SleepSense is built on findings from these published studies:

Why babies get sleepy

Adenosine, caffeine, and sleep-wake regulation: state of the science and perspectives - Journal of Sleep Research, 2022

How a chemical called adenosine builds up during awake time and creates the need for sleep.

How babies develop daily rhythms

Development of the circadian system in early life: maternal and environmental factors - Journal of Physiological Anthropology, 2022

Babies start developing regular sleep-wake cycles between 6-12 weeks, getting more predictable over time.

How much babies sleep at each age

Longitudinal Study of Sleep Behavior in Normal Infants - JCSM, 2014

Nighttime sleep grows from about 6.7 hours at 1 month to 11.7 hours at 7 months. Daytime naps go from 3-4 per day at 3 months down to about 2 by 12 months.

How sleep changes in the first two years

The Architecture of Early Childhood Sleep Over the First Two Years - Maternal and Child Health Journal, 2023

Total sleep time goes from about 13.3 hours at 1 month to 12.2 hours at 12 months. Daytime sleep shrinks from 6.5 hours to just 1.5 hours by 18 months.

Why nap timing matters for nighttime sleep

Daytime nap controls toddlers' nighttime sleep - Scientific Reports, 2016

When a nap happens during the day has a real impact on how well (and how long) the baby sleeps at night.

Why personalization matters

Every baby is different. How quickly they get tired depends on:

Their unique biology
Their environment (light, noise, activity)
What developmental stage they're in
Their personality and temperament

This is why a standard wake window chart works perfectly for some babies and not at all for others. SleepSense learns what makes your baby tick and adjusts accordingly.

Corrected Age for Premature Babies

Babies born before their due date develop on a different timeline. SleepSense automatically accounts for this when you enter a due date:

A baby born 2 months early at 4 months old gets wake windows for a 2-month-old
The adjustment applies automatically until 24 months
Full-term babies (born on or after due date) are unaffected

Transparency

nappi shows you what's behind every prediction:

What factors went into this specific prediction
How confident the prediction is, and why
Whether it's based on general age guidelines or your baby's own patterns