Why Do Babies Sleep with Their Arms up?

It's Part Of Their Moro Reflex

If you've ever witnessed your sleeping baby suddenly startle and fling their arms up in the air, then you've seen the Moro reflex in action. As explained in a study of the same name, the Moro reflex is a completely normal reflex in full-term babies.

Is it OK for baby to sleep with arms up?

Arms-up position is great for self-soothing. Transition at the first sign of rolling, likely 3-6 months. Please don't put anything in the crib at all—blankets, toys, bumpers, etc. They can't keep them away from their face before one year.

Why does my baby always hold his arms up?

The startle reflex (or Moro reflex) is present in most young infants until they reach around 3 months old. If your baby is startled by a loud noise, large motion, or anything else, you may notice that both arms fly upward or outward before coming down again, often in a jerky fashion.