Four months is the most significant sleep milestone in the first year — and often the most difficult. The 4-month sleep regression isn't a phase that passes. It's a permanent change in how your baby sleeps, and how you respond to it shapes sleep for months to come.
What Changes at 4 Months
Before 4 months, babies cycle primarily through light and deep sleep. At 4 months, their sleep architecture permanently matures to include the full adult cycle: light sleep → deep sleep → REM → back to light sleep. This cycle takes 45–50 minutes. At the end of each cycle, babies partially rouse. Previously they'd slip back under easily. Now they wake fully — and if they need help to fall asleep (feeding, rocking), they'll need that help again at every cycle boundary.
Wake Windows at 4 Months
By 4 months, wake windows have stretched to 1.5–2 hours. Many babies are moving toward a 4-nap schedule, though some are still on 5.
Sample Schedule
7:00 AM — Wake, feed
8:30 AM — Nap 1 (45–60 min)
10:00 AM — Wake, feed
11:30 AM — Nap 2 (45–60 min)
1:00 PM — Wake, feed
2:30 PM — Nap 3 (30–45 min)
4:00 PM — Wake, feed
5:15 PM — Catnap (20–30 min)
7:00 PM — Bedtime routine begins
7:30 PM — Bedtime
Night — 1–3 feeds still expected
The Most Important Thing You Can Do
Start practising drowsy but awake at every sleep. This means putting your baby down when they're sleepy but still conscious — not fully asleep. It's the single most important habit to establish at this age. It's hard at first. It gets easier within 1–2 weeks and pays off for months.