Science of baby sleep

The three forces

Three systems run your baby's sleep, and they work as a set.

Sleep pressure is the sleepiness that builds while your baby is awake. Enough awake time means a body that's ready to sleep.

Total daily sleep is how much they need across naps and nights to feel rested.

The body clock is the internal timer, set by light and dark, that decides when sleep should happen. It is immature at birth, starts responding to light at around eight or nine weeks, and strengthens slowly between 12 and 16 weeks.

When one of the three is off, sleep gets harder. When all three line up, your baby's body knows what to do without being told.

Sleep pressure, and how long they can stay awake

Sleep pressure is the drive to sleep, and it builds the longer your baby is awake, like a balloon filling. Sleeping clears it, which is what makes them alert again afterwards.

It builds faster in babies than in adults, which is a good part of why they nap so often. And it runs separately from the body clock, though the two work alongside each other.

What builds it is time spent awake. An active, stimulating stretch builds it faster than a quiet one, but nothing substitutes for the time itself.

A wake window is how long your baby can comfortably stay up between sleeps: long enough for pressure to build, not so long that they tip past it. Roughly, by stage:

The clock, and what sets it

Every living thing keeps time. The circadian rhythm, from the Latin for "about a day", runs on a cycle slightly longer than 24 hours, and it sets far more than sleep. It decides when your baby is alert and when they're sleepy, and when they get hungry and how they digest. It sets when hormones are released, how body temperature rises and falls through the day, and how the immune system works. It does not set anything fixed, like eye color. It is a scheduler, not a blueprint.

Because the cycle runs slightly long, something has to pull it back into line with the world each day. Those cues have a name: zeitgebers, German for "time givers".

Light and dark are by far the strongest. After that come feeds arriving at roughly the same times each day. Then the shape of the day: play, bath, the things you always do. And then the natural drop in temperature at night.

Light and dark

Light is the strongest cue there is, and darkness matters just as much. Most people underestimate both, so here is the scale.

Which is the practical answer for a night feed: a small red or amber lamp, rather than the overhead light or a phone screen.

How the day sets up the night

Longer nights arrive once the body clock has matured. Enough to eat in the day and the right amount of daytime sleep both help.

Enough calories in the day means fewer wakings from hunger. Too much daytime sleep leaves too little pressure for the night. Too little leaves them overtired, which wakes them more.

In early infancy, waking to feed at night is entirely normal, and a small stomach is most of it.