Babies need a lot of sleep. Truer words have never been spoken, but what confounds new and even veteran parents is the question of how long they should sleep at a stretch. And since babies can’t exactly tell you they’re tiredor not tiredhow do you know if you should put them down or keep them up?

Although it’s an imperfect science at best, you can actually discern if your baby is overtired and needs to be soothed and settled to sleep; or if he’s undertired, and will only fight Mr. Sandman (and frustrate you). Here’s what you need to know:

What is overtiredness? It can be tougher to spot in an infant, but if you have a toddler, you know what overtiredness looks like: crankiness, unreasonableness, and sometimes physical clumsiness. Is your toddler tossing toys in frustration or walking, literally, into walls? Yup, tired. It’s helpful to know that, in fact, younger babies exhibit the same signs of overtiredness (minus walking into walls). But they do become unreasonable. Something that interested them just five minutes ago, like a toy or some music, now brings frowns, batting away, crying. That’s a clear cue for overtiredness.

The issue is that not all babies give you much of a warning, going from zero to overtired in no time flat. If you’re lucky, though, you have a baby who short-circuits the whole overtired scenario by just…falling asleep wherever he or she is at the time. But here’s what to look for:

- Fussiness and the kind of crying you know isn’t about hunger or a dirty diaper
- A “second wind,” when the baby seems a bit too active
- A baby who’s easily frustrated

What is undertiredness?  The hardest part of identifying if your baby is undertired—not quite tired enough to go down for a nap—is that its signs can mimic overtiredness. You try to settle him to sleep in what is normally a nap or nighttime sleep window…and he resists. The problem with trying to compel an undertired baby to go to sleep is that you may well succeed…and then end up with a baby who takes a much shorter nap than usual.

You know you have a classic undertired-baby situation when you confidently whisk your baby off for a nap when you’re sure he’s ready for it, and then spend 45 minutes trying to convince Mr. Bright Eyes that, no, it’s actually not playtime anymore. What a waste of time for you both! You may also end up with a child who associates naptime with struggle.

If you find this happening on a regular basis, you have an undertired baby and the best thing you can do is not compel sleep, but just wait a bit longer before trying again. Think about it this way: If you tried over several days or weeks to get baby to sleep at, say, noon, and it took until 1 p.m. to finally get him to settle, why not just wait until 1 p.m. next time? It could just be that, as he gets a bit older, he’s better able to remain alert, interested, and engaged and can go longer between naps.

A final point about undertired babies: Pay attention to how much activity and stimulation your baby gets while awake. Perhaps more activity—daily walks in the fresh air, for example—is warranted to be sure your baby is sufficiently tired.

Just watch that he’s not gettingyou guessed it!overtired.