LullsongThe 4-month sleep regression: what’s actually known
Updated 24 October 2026 · Written by the Lullsong team. General information, not medical advice.
Short answer
No study defines a “4-month sleep regression”. Research does show that a baby’s body clock develops quickly in the first months, and that night waking doesn’t fall in a straight line: many babies wake more for a while. If sleep gets worse around four months, it is usually a phase, not something you caused.
Where the idea comes from
The term comes from parenting books and sleep coaching, not from research. It describes something many parents notice: a baby who had longer stretches suddenly wakes more often, around three to five months.
That timing fits with real development. Clear day–night rhythms in sleep and hormones generally appear after about two months of age (Rivkees, 2003). Sleep is reorganising during these months, so changes are expected.
What studies of night waking show
A review of 34 studies (Galland et al., 2012) found an average of 0.8 night wakings at three to six months and 1.1 at seven to eleven months. On average, waking did not keep falling through the first year.
Individual babies vary even more. In a study of over 1,200 children (Weinraub et al., 2012), a third still woke most nights at six months, and most of them woke less over the following year.
What helps
- Keep a short, calm routine before sleep, the same every night.
- Daylight in the day and dim, quiet nights. In premature babies, a regular light–dark cycle helped rhythms develop earlier (Rivkees, 2003).
- Check the basics: hunger, illness and teething can all disturb sleep.
- Call your paediatrician if your baby has a fever, feeds poorly, or breathing seems difficult.
See also sleeping through the night: what’s normal.
Sources
- Rivkees SA. Developing circadian rhythmicity in infants. Pediatrics 2003;112:373-381. https://doi.org/10.1542/peds.112.2.373
- Galland BC, Taylor BJ, Elder DE, Herbison P. Normal sleep patterns in infants and children: a systematic review of observational studies. Sleep Med Rev 2012;16:213-222. https://doi.org/10.1016/j.smrv.2011.06.001
- Weinraub M, Bender RH, Friedman SL, et al. Patterns of developmental change in infants' nighttime sleep awakenings from 6 through 36 months of age. Dev Psychol 2012;48:1511-1528. https://doi.org/10.1037/a0027680
More sleep guides
- Wake window calculator
- Nap schedule maker
- Safe sleep: the essentials
- Sleeping through the night: what’s normal
- Wake windows: what the research actually says
- Is white noise safe for babies?
- When to stop swaddling
- Room temperature and what to wear
- Contact naps: what’s safe and what’s known
- Dream feeds: do they work?
- Lullabies and singing: what the research shows
Lullsong
Lullsong plays soft white noise and lullabies for naps and bedtime, with a 30-minute timer on by default.