Is it a sleep regression? What's actually going on at 4 months, 18 months and 2 years

Short answer: Probably not a regression in the sense of going backwards. Raising Children Network says "regression" isn't a good word for what are normal developmental changes, and large datasets show no single month with a spike in sleep problems. New skills like sitting, crawling and walking can bring a few weeks of extra night waking, and bedtime resistance often peaks around 18 months.

You're not imagining it. Sleep that was going well can fall apart for a few weeks, and it's exhausting. But the word "regression" makes it sound like your child is going backwards, and the evidence doesn't really support that.

What the research says

Raising Children Network, the Australian Government-funded parenting site, says plainly that "'regression' isn't a good word" for these changes. Shifts in sleep across the first year are "developmentally normal" and part of a slow move towards mature sleep over the first five years ^[1].

Professor Jodi Mindell, a paediatric sleep researcher, told BBC Future she has looked at every month of sleep in the first two years and found no single month with a sudden peak in sleep problems. "It's just different babies at different times" ^[2]. That's expert opinion reported in journalism, not a peer-reviewed analysis, so treat it as a strong hint rather than proof.

What is in the peer-reviewed literature: babies who have just started crawling or walking have more night waking and more broken sleep, and the onset of sitting, cruising and walking brings more awakenings, "often referred to as 'sleep regression' by parents" ^[3].

Around 4 months

Sleep is maturing in the first six months. Another researcher quoted by the BBC says this change can happen any time in that window and isn't a step backwards ^[2]. If your baby is under 6 months, the focus stays on safe sleep and comforting them to sleep. Structured sleep-training methods haven't been shown to help at this age ^[4].

Around 18 months

Wanting to stay up with the family is the most commonly reported toddler sleep problem. It "peaks around 18 months and improves with age" ^[5]. A routine of a few quiet things, about 20 minutes long, at the same time each night is what child health professionals almost always suggest first ^[6].

Around 2 years

Night terrors are most common between 2 and 4 years. Stay calm, keep your child safe, and don't try to wake them ^[7]. Toddler sleep also reflects growing independence, testing limits and wanting reassurance ^[7].

What usually helps

Keep the routine the same, even when it feels like nothing is working.
Give lots of daytime practice for new skills like standing and walking.
Settle the way you'd like to keep settling, rather than starting a habit you'll want to undo later.
If you've had a few rough weeks and you're running on empty, a child health nurse is free, and a sleep consultant can make a plan for your family.

Sources

Raising Children Network. Baby sleep patterns at 2–12 months. raisingchildren.net.au/babies/sleep/understanding-sleep/baby…
Ruggeri A. The science of healthy baby sleep. BBC Future, 9 February 2022. bbc.com/future/article/20220131-the-science-of-safe-and-heal…
O'Connor C et al. Sleep and infant development in the first year. Pediatric Research, 2026. nature.com/articles/s41390-026-04780-4
Douglas PS, Hill PS. Behavioral sleep interventions in the first six months of life do not improve outcomes for mothers or infants: a systematic review. J Dev Behav Pediatr 2013;34(7):497–507.
Raising Children Network. Sleep patterns and cycles in babies, kids and teens. raisingchildren.net.au/toddlers/sleep/understanding-sleep/ab…
Raising Children Network. Responsive settling: 6–18 months. raisingchildren.net.au/babies/sleep/settling-routines/respon…
Raising Children Network. Toddler sleep guide. raisingchildren.net.au/toddlers/sleep/understanding-sleep/to…

Quick questions

Is the 4-month sleep regression real?

Sleep does change in the first six months as it matures towards adult-style cycles, but researchers say the change can happen any time in that window and isn't a step backwards. There's no fixed date to brace for.

Why is my 18 month old suddenly fighting bedtime?

Bedtime resistance is the most common toddler sleep problem parents report, and it peaks around 18 months, then improves with age. A short, predictable routine at the same time each night is the usual first step.

How long do these sleep changes last?

Waking linked to a new motor skill usually settles once the skill is practised. If things haven't improved after a few weeks, or you're worried about your child's health, talk to your child health nurse or GP.