The term is not a medical one

"Sleep regression" does not appear in NHS, HSE or American Academy of Pediatrics guidance. It is a parenting term, not a diagnosis, and there is no published definition of an 18 month regression, no agreed duration and no evidence base for a fixed timetable.

That does not mean parents are imagining it. Sleep really does deteriorate for a lot of children around a year and a half, and it deteriorates for reasons that are easy to name. Knowing which of those reasons applies to your child is more useful than knowing how long a supposed regression is meant to last.

What is actually changing at 18 months

Separation anxiety peaks. The AAP notes that many toddlers "skip separation anxiety in infancy and start demonstrating challenges at 15 or 18 months of age", and that separations are hardest when children are hungry, tired or unwell. Bedtime is the longest separation of the day and it happens when a child is at their most tired.

The nap is under pressure. Many children move from two naps to one around this age, and the transition is rarely clean. Too much day sleep pushes bedtime later; too little produces an overtired child who fights sleep and wakes early. Weeks of oscillating between the two is normal.

Language and movement are surging. Children who are practising new skills practise them in the cot. A child standing up shouting new words at 11pm is doing something developmental, not something wrong.

Independence has arrived. The HSE's framing applies: behaviour is a child's way of expressing what they need, and young children "do not yet have the skills to fully manage their emotions." At 18 months a child has discovered they can object, and bedtime is the most obvious thing to object to.

Molars, colds and any change in the household — a new nursery room, a house move, a parent away — land on top of all this. Often several things are happening at once, which is why it feels like a single event.

What the guidance says to do

The NHS advice for a child who will not go to bed is specific and unglamorous.

Decide what time you want your child to go to bed.
Start a winding-down routine around 30 minutes before they usually fall asleep, and bring bedtime forward by 5 to 10 minutes each week (15 minutes if they are in the habit of going to bed very late) until you reach the time you want.
Set a limit on how long you spend at bedtime — "for example, read only 1 story, then tuck your child in and say goodnight."
Give them their comforter, dummy or favourite toy before settling.
Leave a dim light on if needed.
If they get up, keep taking them back to bed "again with as little fuss as possible."
Be consistent, and expect to repeat the routine for several nights.

For night waking, the NHS advice is to "be as boring as possible to avoid exciting them and leave lights off", and to avoid long afternoon naps. It also advises no screens in the 30 to 60 minutes before bed.

The AAP adds two points that are directly relevant at this age. First, let a child take a favourite thing to bed — a teddy, special blanket or toy — which "often helps children fall asleep, especially if they wake up during the night", provided it has no choking hazards such as buttons or loose ribbons. Second, deal with the requests before bedtime rather than during it: "take care of your child's needs before bedtime so they don't use them as reasons to avoid going to sleep."

The going-back-in problem

The AAP is unusually direct about repeated check-ins: "keep in mind that children's main goal is to get you to appear. So if you appear for any reason, even just for checking, they will expect you to come each time they call out." Its advice is that once you know a child is safe and well, there is no need to go into the room while they work out how to get back to sleep.

That is not the only reasonable position — plenty of families settle their toddler in the room and that is a legitimate choice. The consistency matters more than the method. The AAP's caution is that "a negative response by a parent can sometimes make a sleep problem worse", and that children need time and opportunity to learn how to get back to sleep.

Sorting out the nap

If bedtime has stretched to 90 minutes and your child is cheerful at 8pm, day sleep is usually the culprit. If they collapse at 4pm and wake at 5am, they are probably not getting enough. The AAP's general guidance is that recommended sleep figures cover total sleep in a 24-hour period, so naps have to be counted in when you are working out whether a child is getting enough.

Practical adjustments during a bad patch: cap the nap rather than dropping it, move it earlier rather than later, and keep waking time in the morning fixed while you experiment with the rest. Change one thing at a time and give it a week.

How long it lasts

There is no published answer, because there is no published condition. In practice, disruption that is driven by a nap transition settles when the new nap pattern settles, usually over a few weeks. Disruption driven by separation anxiety eases as the anxiety does; the AAP notes that by three years old most children clearly understand the effect their protests have, and that consistency and keeping your promise to return are what build the trust that ends it.

The thing that reliably prolongs a bad patch is changing approach every few nights. The NHS's general behaviour advice applies: once you have decided to do something, "continue to do it. Solutions take time to work."

When to ask for help

The NHS says to talk to a health visitor if you need further advice, and that they may have other ideas or suggest an appointment at a children's sleep clinic if there is one in your area. That is a low threshold and worth using — sleep deprivation in parents is a health issue in itself.

Separately, get advice rather than waiting if the poor sleep comes with heavy snoring most nights, pauses in breathing, gasping or choking noises, or daytime exhaustion, because those point to a breathing problem in sleep rather than a behavioural one. And mention it if a child seems in pain at night, or if sleep has changed suddenly alongside any loss of skills they previously had.

18 Month Sleep Regression: Is It Real, and How Long?