The startle reflex (Moro reflex) during sleep: is it normal?
Baby startles, flings their arms out, and sometimes cries while falling asleep. This primitive reflex, present from birth, is completely normal.
The Moro reflex, also called the startle reflex, is a primitive reflex present in every newborn from birth. It's triggered by a sensation of falling, a sudden noise, or even just being laid down in the crib: baby's arms fling outward, hands open, then pull back in toward the body, sometimes with crying.
This reflex is thought to be an evolutionary leftover that once helped infants instinctively grab onto a caregiver if they lost their balance. It's completely involuntary and doesn't signal fear or particular discomfort, even though it can wake baby just as they were starting to drift off — hence the nickname 'sleep startle.'
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Safe swaddling, which keeps the arms close to the body, is often used to soften these startles and ease falling asleep, as long as the usual safety rules are followed (hips free to move, baby on their back, and swaddling stopped once they start rolling). Staying in close physical contact while laying baby down can also help.
The Moro reflex normally fades between 3 and 6 months as baby's nervous system matures and more voluntary movements take over. If it persists past 6 months, is asymmetrical (only one side of the body reacts), or is absent on one side, it's worth mentioning to your healthcare provider at your next regular check-up.
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This information is general and does not replace advice from your healthcare professional (doctor, midwife, nurse, pharmacist). Every situation is unique: always check with them before making a decision. In an emergency, call 911 or your local emergency services.
