eli5: why do babies cry when they’re overtired when they can just sleep?

Question

eli5: why do babies cry when they’re overtired when they can just sleep?

In: 2

Answer 1

I mean, I cry when I’m overtired too. But the actual answer is cortisol, which is a hormone that’s produced when you’re stressed. It keeps you awake and alive when you’re in dangerous situations – it’s basically the fight-or-flight hormone.

Babies have an optimal sleep window, where they’re both tired and relaxed enough to sleep deeply and well – they feel sleepy and safe. When they’re kept awake too long it triggers cortisol production, which makes it much harder for them to settle down and sleep. It’s the same reason adults often have trouble sleeping when they’re stressed out.