What is the difference between a growth spurt and a sleep regression?
Both describe periods when a previously settled baby becomes unsettled, and parents frequently use the terms interchangeably. They describe different things, and one of them has considerably better evidence than the other.
Growth spurts. Periods of accelerated physical growth. In infancy these are associated with:
Increased feeding, sometimes near-constant — cluster feeding.
Fussiness and difficulty settling.
Extra sleep in some cases, or disrupted sleep in others.
Commonly cited around 2–3 weeks, 6 weeks, 3 months and 6 months, though the specific timings are folk wisdom rather than firmly established — research on infant growth finds it occurs in irregular bursts, but the popular calendar of spurts is not well supported.
For breastfeeding, this matters practically. Increased feeding raises supply through demand, so a spurt is the mechanism by which supply adjusts. It is very commonly misread as "my milk has dried up", which leads to unnecessary supplementation.
Sleep regressions. Periods when an infant who was sleeping reasonably starts waking frequently, resisting naps or fighting bedtime. Commonly attributed to 4 months, 8–10 months, 12 months, 18 months and 2 years.
The 4-month one is the best supported and is genuinely different — it reflects a permanent change in sleep architecture. Newborn sleep has a simple structure; around four months it matures toward adult-like sleep cycles with distinct stages and brief arousals between them. Babies who could not previously wake between cycles now can, and if they fall asleep only while fed or held, they need the same conditions restored each time.
This is a developmental change, not a regression — the sleep has matured, and the term is misleading.
Other "regressions" typically coincide with developmental leaps — rolling, crawling, standing, separation anxiety, language bursts — where a baby practises new skills at night.
Both are temporary, typically lasting days to a few weeks.