Why Won't My Baby Sleep? The Real Reasons — and How to Fix Them

It's late, you've tried everything, and your baby is still wide awake (or wide awake again). When you're this tired, "why won't my baby sleep?" stops being a question and starts being a plea. The honest answer is that "won't sleep" is a symptom, not a cause — and once you find the cause, the fix is usually much smaller than the exhaustion makes it feel.

Here are the real reasons, and how to tell which one you're dealing with tonight.

The usual suspects

• Overtired. The most common culprit. Awake too long, stress hormones rise, and an overtired baby fights sleep, catnaps, and wakes early. Tell-tale sign: frantic, hard-to-settle, worse the longer you wait.
• Undertired. The opposite — put down before enough sleep pressure has built. Tell-tale sign: happy, chatty, "playing" in the cot, or a bedtime that drags on for an hour.
• Wake window slightly off. Even 20–30 minutes too long or too short throws the whole settle out. See wake windows by age.
• Hunger. A growth spurt or a too-small last feed means they genuinely can't settle. Cluster feeding in the evening is normal in the early months.
• A false start. Baby goes down beautifully, then wakes 30–45 minutes later, fully upset. This is almost always a timing issue (over- or under-tired at bedtime), not a habit.
• A regression or developmental leap. Around 4 months, and again later, sleep gets temporarily harder while the brain matures or a new skill brews.
• Discomfort. Teething, a cold, trapped wind, or silent reflux — see colic and reflux support.
• The sleep environment. Too bright, too warm, too stimulating, or too quiet after the womb's constant whoosh.
• A strong sleep association. Baby only knows how to fall asleep one specific way (fed, rocked, held) and needs it recreated at every wake.

How to work out which one it is

Play detective with a few quick questions:

• When did they last sleep, and for how long? Long gap or skipped nap → likely overtired. Barely any awake time → undertired.
• When did they last feed, and how much? Long gap or a snacky feed → check hunger.
• Did they go down fine, then wake fast? That's a false start — usually fix the bedtime timing, not the baby.
• Anything new this week? New skill, tooth, cold, travel, or roughly the right age → suspect a regression/leap.
• Are they waking happy or upset? Happy at 5am with hours of day sleep behind them → often too much day sleep or an early bedtime. Upset and frantic → tiredness or discomfort.

Nine times out of ten, the answer is timing — the wake window or bedtime is a little off — and a small adjustment for a few nights resets everything.

The hard part: it changes constantly

The frustrating truth is that the reason changes as your baby grows. What worked last month stops working; the "right" wake window drifts; a leap arrives uninvited. Reading it correctly, night after night, on no sleep, is genuinely hard.

That's why we built OBubba. Instead of leaving you to guess, OBubba's sleep engine reads your real logs and *tells you why your baby isn't sleeping — overtired, undertired, false start, regression, hunger, day-sleep surplus — and what to change tonight. It learns your* baby's patterns and predicts the next nap and bedtime, like a sleep consultant in your pocket, never prescriptive and always kind. See how on our baby sleep consultant app page.

You're not doing anything wrong. Your baby just can't tell you what's off yet — and finding the why is the whole game.