How SleepSense Works

The formula

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

SleepSense age reference table

These are the age-based values SleepSense uses before it learns your baby's patterns. Wake-window cells show the minimum, typical, and maximum. When babies are moving from one nap count to another, the table also shows the range for each count.

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 because 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 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% because 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:

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:

The research behind it

SleepSense is built on findings from these published studies:

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