Your baby was sleeping better, and now they are not. That is what most parents mean by a sleep regression: broken nights, short naps or long bedtimes in a baby who had been settling well.
It is a parenting term rather than a medical one, and it does not appear in NHS guidance. Parents often report a few weeks, but no study has measured how long a sleep regression lasts.
What Changes at Each Age
At 4 months there is a likely reason. From about 4 months, deeper sleep separates into distinct stages and starts to look more like adult sleep. That change is permanent. What improves is your baby's ability to join one cycle to the next without you.
The other ages are less tidy. Several ordinary changes land in the same few weeks, and sleep shows it first. At 6 months that often means rolling both ways, starting solids, and a schedule that no longer fits. At 8 months, crawling or bottom shuffling, which the NHS puts at 7 to 10 months, plus pulling up to stand and stronger separation anxiety. At 12 months, first steps, first words, and often a new nursery or childminder, while most babies still need two naps. At 18 months, tantrums and the canine teeth, both of which the NHS puts at about this age, with one nap now carrying the whole day.
What Holds True Throughout
Watch the cause rather than the calendar. Sleep usually settles when the thing behind it does: a tooth comes through, the nap moves, a new skill stops being exciting.
The same few things help at every age. Keep the wind-down short and in the same order every night. Change one thing at a time, then give it several days. Bring bedtime forward after a bad nap day. Pick a response you can repeat at 3am for a fortnight.
If you use wake windows, the awake time between sleeps, treat the age ranges as a starting point. NHS national guidance does not use the term, and the numbers come from sleep-coaching practice. Our wake windows guide and baby sleep tips go further.
If broken sleep drags on with no sign of improving, or you are struggling to cope, speak to your health visitor or GP.