"Regression" is a misleading word. Sleep does not go backwards — it reorganises, usually because something in your baby's brain or body has just moved forwards. The disruption is real, but it is a sign of progress and it is temporary.
The common windows:
4 months (often starts 3-4). The big one, and the only true structural change. Your baby's sleep architecture matures from newborn-style sleep into adult-like cycles with distinct stages. They now surface fully between cycles, which means every 45 to 60 minutes there is a chance of a full wake-up. This one is permanent — sleep never goes back to the newborn pattern. What settles is your baby's ability to link cycles. Typically two to six weeks of disruption.
6 months. Often driven by teething, rolling, and the beginnings of separation awareness. Usually one to two weeks.
8-10 months. Crawling, pulling to stand, and a big leap in separation anxiety. Babies frequently wake to practise new motor skills. Two to six weeks.
12 months. Walking, first words, and often a premature attempt to drop to one nap. One to three weeks.
18 months. Separation anxiety plus emerging autonomy and molars. Two to four weeks.
2 years. Imagination, night fears, and the transition out of a cot. Variable.
What actually helps:
- Protect wake windows. Overtiredness makes every regression worse. If nights are broken, daytime sleep matters more, not less.
- Keep the routine identical. The wind-down sequence is the signal. Now is the worst time to change it.
- Add consistency, not novelty. Introducing a brand new sleep association mid-regression usually backfires.
- Give practice time in the day. If they are waking to roll or pull up, let them do it extensively while awake.
- Continuous low-frequency sound helps at the cycle transitions — the moments a half-waking baby either resettles or comes fully awake.
What to avoid: starting sleep training mid-illness or mid-teething, changing rooms or cots during a regression, and dropping a nap because one day went badly.
See your doctor if disrupted sleep comes with fever, ear-pulling with distress, poor feeding, breathing changes, or if it lasts far longer than the windows above without improving.
More on the underlying pattern: why babies cry in their sleep.