15-Month Sleep Regression: Why It Happens and What Helps

Your 15-month-old has most likely stopped sleeping well because several developmental forces are converging at once: growing confidence in walking, an early language explosion, a peak in separation anxiety, and — for many families — the looming pressure of a major nap transition. These changes tend to cluster between 14 and 16 months, and when they collide, sleep takes a hit.

If you came through the 12-month sleep regression, you might have expected a calmer stretch by now. And for some families, it was. But the 15-month mark brings its own set of pressures — less dramatic than what came before, but no less disruptive.

Walking is probably the biggest factor. At 12 months, many babies were just learning to pull up and cruise. By 15 months, walking is becoming a genuine skill — and the brain is still consolidating it. Research shows that motor skill development is processed during sleep, particularly during REM. So your toddler may be physically restless at night, practising movements, or simply too wired from the day's adventures to settle easily.

Language is picking up pace too. Your toddler likely understands far more words than they can say, and the gap between comprehension and expression can be frustrating. That frustration, combined with a growing awareness that bedtime means being apart from you, fuels the separation anxiety that peaks around this age. Your toddler knows you exist when you leave the room — and they want you back.

Add to all of this the fact that many babies are either approaching or in the middle of the 2-to-1 nap transition, and you have a recipe for disrupted nights, refused naps, and early mornings. The good news is that this is temporary. The tricky part is knowing what to do — and what not to do — while it lasts.

Like every so-called sleep regression, this is actually a progression. Your toddler's brain is doing extraordinary things, and sleep is temporarily caught in the crossfire.

The word "regression" implies going backwards, but nothing about your toddler's development is moving in reverse. Their sleep architecture is well established by now — they went through the permanent biological change at 4 months and have been building on it ever since. What is happening at 15 months is a temporary disruption caused by the brain processing new skills, new awareness, and new emotions.

It is worth noting that the 15-month regression is not always recognised as a distinct phase. Many sleep resources group it with the 12-month or 18-month regression. But families living through it know it feels very real. The developmental triggers at 15 months are genuinely different from those at 12 months — walking is more established, language is further along, and the nap transition question is far more pressing.

The disruption can start as early as 14 months or as late as 16 months. Every baby's developmental timeline is different. If your toddler was sleeping well and has suddenly started waking at night, fighting bedtime, or refusing naps — and they are somewhere in the 14-to-16-month window — this is very likely what is happening.

Understanding that this is a sign of progress, not a problem with your toddler's ability to sleep, can genuinely change how you feel about it at 3am. The skills are still there. They are just temporarily overwhelmed.

The 15-month sleep regression typically lasts 2 to 6 weeks, though the exact duration depends on how many developmental changes are happening simultaneously and — crucially — whether the nap transition is part of the picture.

If the disruption is primarily driven by motor development and separation anxiety, it tends to resolve within 2 to 4 weeks as those skills become more established and the anxiety settles. This is similar to the 12-month regression in duration.

If the nap transition is also in play, the disruption can stretch longer. Moving from two naps to one is one of the biggest schedule changes of the first two years, and it rarely happens overnight. Many families experience a messy middle period — some days two naps work, some days one nap works, and some days nothing seems to work. This transitional phase can overlap with the regression and extend the overall period of disruption.

The regression can also last longer if parents inadvertently introduce new sleep associations during the disruption — bringing their toddler into bed, reintroducing a feed that had naturally dropped, or rocking back to sleep at every waking. These responses are completely understandable in the moment, but they can create patterns that persist after the developmental phase has passed.

If sleep has been significantly disrupted for more than 6 weeks with no improvement at all, it is worth speaking to your GP or health visitor to rule out anything else that might be contributing.

The nap transition is often the single biggest factor in the 15-month sleep regression — and the source of most confusion. Many babies start showing signs of readiness to move from two naps to one somewhere between 14 and 18 months, and the timing overlaps directly with this regression.

The challenge is telling the difference between a temporary nap refusal caused by the regression and a genuine readiness to transition. Getting this wrong — dropping to one nap too early — can turn a short-lived disruption into weeks of overtiredness, worse night sleep, and earlier mornings.

There are some signs that suggest genuine readiness for one nap, and some that suggest the regression is the culprit:

• Regression sign: The nap refusal started suddenly, alongside night waking and bedtime resistance. This usually means the developmental leap is driving it, not a schedule change.
• Regression sign: Your toddler refuses the second nap but then becomes visibly tired, cranky, or falls asleep in the car or pushchair before bedtime.
• Transition sign: Your toddler has been consistently taking a long time to fall asleep for the second nap — 30 to 45 minutes — for at least two weeks, and night sleep has remained stable or improved.
• Transition sign: Your toddler can comfortably stay awake for 5 or more hours without becoming overtired or distressed.

If you are unsure, the safest approach is to keep offering two naps for at least another two to three weeks before making any permanent changes. If the second nap is refused, bring bedtime earlier to compensate. An early bedtime — even as early as 6pm — is far less risky than dropping a nap too soon. For a deeper look at the transition itself, see our guide to the 2-to-1 nap transition.

The most helpful thing you can do is stay consistent with your existing routines and resist the urge to overhaul everything in response to a temporary phase.

This is a period of change for your toddler, and predictability is their anchor. The bedtime routine, the sleep environment, the way you respond to night waking — keeping these steady gives your toddler the security to work through the developmental changes without everything else shifting beneath them.

A few general principles that tend to help:

• Keep the bedtime routine consistent. The same steps, same order, every night. Predictability is reassuring when everything else in your toddler's world is changing.
• Avoid introducing new sleep associations. If your toddler was self-settling before the regression, those skills are still there — they are temporarily overwhelmed. Introducing new props or habits now can create dependencies that outlast the regression.
• Keep the sleep environment unchanged. Dark room, consistent temperature, white noise if you use it. This is not the time to make changes to where or how your toddler sleeps.
• Offer plenty of active play during the day. Walking practice, climbing, outdoor exploration. Allowing your toddler to process their motor milestones during waking hours can reduce the need to practise at night.
• Use an early bedtime as your safety net. If naps have been short or refused, bringing bedtime earlier prevents the overtiredness spiral. A bedtime of 6pm to 6:30pm is perfectly fine as a temporary measure.
• Keep night-time responses brief and calm. If your toddler wakes, reassure them, settle them, and keep the interaction low-key. The message is: "I'm here, you're safe, it's still sleep time."

Most families find that once the developmental leap settles and any nap transition stabilises, sleep returns to something close to what it was before. The toddlers who were sleeping through tend to return to sleeping through. The self-settling skills have not been lost — they were temporarily outpaced by everything else happening in that busy brain.

The 15-month sleep regression is a normal developmental phase. But not everything that disrupts sleep at this age is developmental. It is worth knowing when to seek additional support.

Speak to your GP or health visitor if:

• The disruption has continued for more than 6 weeks with no improvement at all — not even small signs of progress
• Your toddler seems to be in pain — inconsolable crying that is different from normal unsettled behaviour, particularly if accompanied by a persistent fever
• You notice breathing changes during sleep — persistent snoring, gasping, or pauses in breathing
• Your toddler is losing weight or refusing food alongside the sleep disruption
• Developmental milestones seem to be stalling rather than progressing — not walking, not attempting words, not engaging
• You are struggling. Fifteen months of disrupted sleep is a long time. If it is affecting your mental health, your ability to cope, or your relationships, please seek support. Your GP, health visitor, or the PANDAS Foundation can help.

If you are concerned about your toddler's health, speak to your GP or health visitor. This is sleep support, not medical advice — and anything that could have a medical cause needs professional assessment first.

For the vast majority of families, this regression is a temporary chapter in a much longer story. Your toddler is building walking confidence, developing language, and learning about the world at a pace that is genuinely extraordinary. The sleep disruption is the short-term cost of that progress, and it does pass. But if you would like personalised support to navigate the specifics — particularly around the nap transition question or how to respond to night waking without creating new habits — that is exactly what one-to-one guidance is for.