Just when a sleep pattern finally feels settled, it falls apart again - and if you've been through this before, you already suspect it's 'a regression,' even if you can't quite place why they keep happening on a rough schedule of their own.
Sleep regressions aren't random. Most cluster around specific developmental ages, each driven by a different leap. Here's the full timeline.
The 4-month regression
Around 3-5 months, sleep architecture matures into adult-like cycles with lighter and deeper stages, causing more frequent brief wake-ups between cycles. This is considered a permanent shift rather than a passing phase - what improves is your baby's ability to link cycles back together, not the underlying pattern reversing.
The 8-month regression
Around 8-10 months, major developmental leaps - crawling, pulling to stand, object permanence, and often separation anxiety - tend to disrupt sleep simultaneously. Babies at this age may practise new physical skills in the cot, and separation anxiety can make settling without a parent present genuinely harder for a stretch.
This regression is often shorter-lived than the 4-month shift, commonly easing within 2-4 weeks as the new skills consolidate.
The 12-month regression
Walking, language explosion, and often the transition toward one nap a day converge around this age. A nap transition attempted too early can look identical to a regression, so it's worth checking whether this is a temporary disruption or a genuine nap-count change (see our guide on nap transitions for the difference).
The 18-month regression
Growing independence, stronger opinions, and the beginnings of testing boundaries (including at bedtime) show up around 18 months, alongside continued language development. This one often looks more behavioural - bedtime resistance, calling out, wanting a parent to stay - than the earlier regressions, which were more purely physiological.
What helps across all of them
- Keep the routine consistent even when sleep itself is messy - predictability is the anchor during a regression
- Expect a temporary step back and resist the urge to overhaul the whole approach in response to a few rough weeks
- Offer extra reassurance and connection during the day, which can ease clinginess and separation anxiety at bedtime
- Give it time before troubleshooting further - most regressions resolve within 2-6 weeks as the underlying developmental leap settles
The pattern worth remembering
Every regression on this timeline is a sign of development happening on schedule, not a sign that sleep is broken or that something needs fixing long-term. They're temporary responses to genuine leaps forward.
Ride out the rough stretch with the routine intact, and sleep almost always finds its way back - often to a better place than before, once the new skill has settled in.