A baby who had started sleeping more predictably may suddenly wake more often or struggle to settle. This is commonly called a sleep regression. The phrase is useful shorthand, but it is not a medical diagnosis and it does not happen on the same schedule for every baby.
What does 'sleep regression' mean?
Parents use the term to describe a period when a baby's sleep becomes less settled after seeming to improve. Health authorities do not define it as an illness or a developmental stage with a fixed start and end date. It is a description of a change, not an explanation for every sleep problem.
What changes around four months?
Sleep continues to mature throughout the early months. Day and night become more distinct, and the way babies move through sleep cycles changes. That can affect how often a baby wakes or needs help settling, but the timing and length of the change vary widely.
What can you do?
Keep following safe-sleep guidance, use a simple and familiar bedtime routine, and note when the change began. If your baby is sleeping very little, feeding less, or showing other symptoms that concern you, speak with your pediatrician rather than assuming it is only a regression.