Sleep drive is lowest before dawn
This is the underlying reason early waking is so common. After nine or ten hours asleep, the pressure to keep sleeping has largely been paid off, so it takes less to wake your baby fully — and more to get them back down.
Light
Even a thin strip of dawn light around a curtain can be enough at this hour, especially in summer. A room that looks dark to you at 8 p.m. may be bright at 5 a.m.
Overtiredness
It sounds backwards, but a bedtime that's too late or a final wake window that runs long often leads to an earlier morning, not a later one. Overtired babies tend to sleep more lightly towards the end of the night.
A first nap that comes too early
If your baby wakes at 5 and goes down for a nap at 6:30, that nap works like a continuation of the night — which quietly rewards the early start. Holding the first nap to an age-appropriate wake window is one of the most effective levers.
Too much daytime sleep, or simply the math
A baby who naps longer than typical for their age, or who goes to bed at 6 p.m. and sleeps eleven hours, may simply be done by 5. Compare daytime sleep against the table below before changing anything else.
A habit feed or noise
A feed at 5 a.m. every morning can become something your body clock — and your baby's — expects. Birds, traffic, and someone leaving for work are common culprits too.