How to Babies Sleep? They follow the Ultadian Rhythm.
Every person has a couple of internal clocks that rule their bodies various cycles (example, the well known 24-hour circadian cycle). For babies and parents, the Ultadian is in charge.
The Ultadian Rhythm runs in 90 minute increments. Meaning, the body ramps up and is alert for 90 minutes before slowing down.
For adults, this is time to unplug. For babies it is the time to sleep. (and time for parents to catch up on sleep!)
This means that after being awake and alert for 90 minutes, your baby is biologically ready to sleep. This 90 minute routine is the ideal sleep/wake cycle for babies and it will help your them experience more restful sleep and better periods of alertness.
While this rhythm exists in all humans, babies will evolve as they age and their 90 minute sleep pattern will stretch out. Eventually they'll be awake for 3 hours, 4 hours, etc. and the stretches of sleep will also increase.
Don't be fooled and think this cycle will make your baby sleep - not true.
Since the Ultadian rhythm sets the period of wakefulness, your baby will still need help falling asleep. This will not happen automatically. They will still need your help settling (learning to self-soothe) and recognizing time to sleep.