What is actually happening

Start with the label. "Sleep regression" is a term from parenting and sleep-consulting culture, not from clinical guidance. The NHS does not use it. What the NHS describes instead is the ordinary instability of the first two years: "As your baby grows, their sleep pattern will change. It can feel as though every time you have settled into a routine, it changes again."

The Sleep Foundation, which does use the term, places the common ones at around four months, and then at six, eight, twelve and eighteen months. Its explanation for the four-month version is developmental rather than pathological: "At around 4 months, a baby's brain and body are developing rapidly, and the process of forming and linking different areas of the brain and nervous system may create instability in sleep." At the same age, "a baby's sleep starts to consolidate, meaning that they begin to sleep for longer periods at a time".

That is the paradox worth naming. The four-month regression is not sleep going backwards. It is sleep reorganising, and reorganisation is noisy.

What is normal, as a range

Around four months

Signs the Sleep Foundation lists include difficulty falling asleep, "frequent nighttime awakenings" often with crying, "irritability upon waking" and "reduced total sleep time" across both day and night. Naps often take the hit first — shorter, harder to start, and followed by an overtired baby. On duration: "In most cases, sleep problems only last for a few days to a few weeks, but this depends in part on fostering good sleep habits."

Around eight months

Here the driver is what your baby can suddenly do. The Sleep Foundation notes that "around this age, many infants have started teething and can roll over, sit up on their own, and crawl", alongside "emotional development that can increase separation anxiety" and "greater environmental awareness that spurs overstimulation". Expect difficulty falling asleep, more night waking, fussiness around bedtime, or longer daytime naps with less night sleep. It is described as normally short-lived.

Around twelve months

The listed contributors are "restlessness and overstimulation related to physical growth and increased activity levels", "separation anxiety that builds up with heightened emotional and social development", "teething and associated pain and discomfort", and adjustment to new schedules. On how long: "In most cases, the symptoms of a 12-month sleep regression do not last longer than a few weeks."

The wider band

Set all of that against the NHS's normal ranges. Some babies sleep eight hours a day in total and some eighteen, and both are described as perfectly normal. From three to six months, "some babies may sleep for 8 hours or longer at night, but not all will". By six months a baby "may sleep for up to 12 hours at night", though the NHS adds that teething pain and hunger can disturb it. A regression is often a baby moving within that band rather than falling out of it.

What is not a regression

This is the part that matters most. A disrupted night in a well baby is one thing; a disrupted night in an unwell baby is another, and the distinction is not subtle. Broken sleep alongside a high temperature, poor feeding, fewer wet nappies, fast or laboured breathing, or a baby who is unusually floppy or hard to rouse is not a sleep problem. The NHS is specific that a temperature of 38C or more in a baby under three months, or 39C or more between three and six months, needs an urgent call.

Teething is worth naming separately. The NHS lists "not sleeping very well" among teething signs — but also says teething raises the temperature only slightly, and to less than 38C. Anything above that is not teething.

When to speak to a health visitor or doctor

Call 111, your GP or your local urgent care line straight away if broken sleep comes with a high temperature at the thresholds above, breathing that looks like hard work, feeding much less than usual, dry nappies, or a baby who will not wake properly. Book a non-urgent appointment, or raise it at the nine-to-twelve-month review, if the disruption has run well beyond a few weeks, if your baby seems tired and unhappy through the day, or if you are running out of road yourself. Parental exhaustion is a legitimate reason to ask for help.

What actually helps

Hold the routine steady. Consistency at bedtime, a dark and quiet room, and a predictable nap schedule are what the Sleep Foundation recommends across all three ages.
Get daylight and movement into the day. Daytime activity and exposure to natural light are named specifically for the twelve-month version.
Cut stimulation before bed rather than adding more soothing after it.
Let the new skill be practised in daylight. A baby who has just learned to pull to stand will do it in the cot at 2am. Floor time earlier helps more than a wrestling match at night.
Do not loosen safe sleep to buy sleep. Back to sleep every sleep, a separate cot or Moses basket in your room for at least the first six months, a firm flat mattress, and nothing in the cot that could cover the face. If your baby has started rolling, the NHS says to move them onto their back until they can roll both ways by themselves.
Expect a return, not a cure. Sleep in the first year is a series of renegotiations. Getting through one is normal; so is the next.

Sleep Regressions