The four month sleep shift

Around four months, sleep architecture matures into adult-like cycles with light phases between them. Your baby now surfaces briefly several times a night and notices whether the conditions have changed since falling asleep.

This is developmental, not a regression in the ordinary sense, and it does not reverse. What helps is consistency: the same short wind down, the same room, the same place at the start of the night.

Daytime matters too. A baby who has had floor time, movement and a few minutes of focused play settles more easily than one who has been carried and entertained all day.