Understanding Baby Sleep Cycles: Why They Wake, What’s Normal, and How Sleep Grows With Them
Ever wonder why newborn sleep feels like such a mystery?
That’s because their sleep is vastly different from ours—and for good reason. Newborns are biologically wired for a unique kind of sleep that supports their rapid development. They cycle through just two stages of sleep: active (REM) and quiet (non-REM). Unlike adults, whose non-REM sleep is broken into multiple stages, newborns’ sleep is still maturing.
Around 3–5 months of age, things begin to shift. Babies start entering non-REM sleep first, and by about 6 months, their non-REM sleep becomes more defined—splitting into 3 distinct stages, much like ours. This evolution plays a major role in how they sleep and why things may feel like they’re changing (or getting harder) around the 4-month mark.
Let’s take a closer look:
How Long Is an Infant’s Sleep Cycle?
Newborns cycle through sleep every 40–50 minutes, while adults cycle every 90 minutes. That means more time spent in REM—about 50% of their total sleep (even more for premature babies). This helps explain why they seem to wake so often or why they appear restless in their sleep.
And yes—those tiny wakings between cycles? Completely normal. If they haven’t yet learned to fall asleep independently, they’ll usually call for help. They’re not quite ready to start the day—they just need help connecting one cycle to the next.
As their sleep patterns mature and their biological clock (circadian rhythm) develops, things start to shift. By around 3–4 months, sleep cycles become more predictable and begin to mirror adult-like stages.
Baby Sleep Cycle Breakdown (4+ Months):
Once babies hit that 3–4 month window, a major developmental change takes place. This is when more distinct, adult-like sleep cycles begin to form—and it changes everything.
Stage 1 – Light Sleep (N1): Drowsy, easy to wake (~5–10 min)
Stage 2 – Light Sleep (N2): Breathing evens out, body relaxes (~20 min)
Stage 3 – Deep Sleep (N3): Most restorative, hard to wake (~20–30 min)
REM Sleep: Dreaming stage, lots of brain activity (~20–30 min)
This is also when circadian rhythms kick in and babies begin to understand day from night. But it’s not just about sleep structure—it’s about sleep awareness. Babies are now aware of how they fall asleep. So, if they’re rocked, nursed, or held at bedtime, they may need those same conditions to fall back asleep after a natural waking.
This can lead to more night wakings, shorter naps, and difficulty falling asleep. It’s all normal, and it’s a sign that their brains and bodies are developing beautifully. But it also means that how we support sleep starts to matter more.
REM Sleep (Active Sleep) – Why It’s Important:
Supports brain development and memory consolidation
Helps babies process emotions and experiences
Boosts mood regulation and coping skills
Provides energy to the brain and body
Increases brain activity, heart rate, and breathing
Lacks motor inhibition in newborns (explaining all that movement!)
Non-REM Sleep (Quiet/Deep Sleep) – Why It Matters:
Deepest, most restorative part of sleep
Encourages physical growth and repair
Releases growth hormones
Supports immune function and glucose metabolism
Harder to wake from—this is when longer stretches happen
Are They Asleep… or Awake?
During REM, babies can trick us into thinking they’re awake—grunting, smiling, making faces. Don’t worry, this is completely normal. Their bodies haven’t developed motor inhibition yet (which keeps adults still during REM). By 3–4 months, that changes, and their sleep becomes more still.
In non-REM sleep, babies are quieter, more still, and much harder to wake—this is when those longer stretches happen.
How Much Sleep Do Babies Need?
Newborns: 14–17 hours per 24 hours (~50% in REM)
6+ Months: 12–15 hours per 24 hours (less REM, more deep sleep)
Understanding your baby’s sleep cycles can help you make sense of what’s going on—and remind you that night wakings aren’t a sign of failure. They’re a normal, healthy part of development.
And the good news? As their sleep patterns mature and their ability to fall asleep independently strengthens, their sleep becomes more restful—for them and for you.
