Your baby was sleeping in beautiful four-hour stretches, and then overnight they're waking every ninety minutes. Welcome to what parents call a sleep regression - though the NHS doesn't actually use that term. What's usually happening is that your baby's brain is going forwards, and their sleep is temporarily disrupted by the effort.
The 4-month regression
This one has genuine science behind it. Before four months, babies cycle between only two sleep stages. Around this age their sleep architecture matures to include lighter sleep stages that adults have. The result: more frequent partial awakenings. It's not a failure of your parenting - it's a neurological upgrade with inconvenient timing.
8-10 months
Separation anxiety peaks and mobility explodes. Crawling babies often seem to want to practise their new skills at 3am.
18 months
Language development, growing independence, and the emergence of molars can all conspire against peaceful nights.
What helps
A consistent bedtime routine: bath, pyjamas, book, feed, bed. During the night, keep interactions boring - lights low, voice quiet, no playing. Sleep regressions are temporary. Most resolve within two to six weeks.