Sleep regression ages
A "sleep regression" is a stretch of worse sleep in a baby who had been sleeping better. The name is a bit misleading: most line up with a developmental step forward, not backwards.
| Age | What's usually going on |
|---|---|
| 4 months | Sleep cycles mature; brief wakings between cycles become noticeable |
| 6 months | Rolling and sitting; some babies resist the third nap |
| 8–10 months | Crawling, pulling to stand, separation anxiety, 3→2 naps |
| 12 months | Walking; temporary nap refusal that is usually not ready for one nap |
| 18 months | Independence, separation anxiety, molars |
| 2 years | Big-kid changes, new sibling, potty training, fears of the dark |
The one with a real biological change
Around 3–5 months, a baby's sleep reorganises. Newborns spend about half their sleep in active (REM-like) sleep and fall straight into it. As the brain matures, babies begin to fall asleep through lighter stages, reach deep sleep, and surface briefly at the end of every cycle, as adults do. Nothing "goes wrong"; it's a permanent upgrade that happens to make those end-of-cycle wakings visible.
The later regressions are less about sleep itself and more about what babies are learning: moving, standing, missing you, and asserting themselves.
Try it: how sleep cycles change around 4 months
A simplified picture of a few hours of night sleep. Toggle between the two ages.
Now sleep starts light, dives into deep sleep, and comes back up to a near-waking at the end of each cycle (amber dots). A baby who fell asleep being fed or rocked may call out to get that help back each time.
Guides
- 4 month sleep regression · usually between 3 and 5 months
- 8 month sleep regression · usually between 8 and 10 months
- 18 month sleep regression · usually between 17 and 20 months
Regression questions
What ages do sleep regressions happen?
The most commonly described ages are around 4 months, 8–10 months, 12 months, 18 months and 2 years. Only the 4 month change has a clear biological cause for every baby; the others line up with developmental leaps and vary widely.
How long do sleep regressions last?
Most families find the worst of it passes in 2–6 weeks. Keeping routines consistent usually shortens it.
When should I call the doctor?
Contact your pediatrician if your baby has a fever, seems unwell, is feeding poorly, snores loudly or pauses in breathing, or if sleep problems go on for more than a few weeks.