Kids ask a lot of questions.

mental_floss has answers. Today we launch WHY?, our new series for kids and parents. We'll tackle all types of questions children have about how the world works by providing science-based, kid-friendly content. Our answers are written with early readers (ages 4–7) in mind, but we think they're interesting—and educational—for everyone.

Have a question? Send it to why@mentalfloss.com. 

Sleep is a must for all animals, including you. You must sleep to live. When we sleep it may seem like we’re not doing much. But this is when our bodies are busy growing, healing, and learning—especially our brains. They make sense of the lessons, games, words, feelings, and thoughts we had during the day. 

We have something inside our bodies called a circadian (sir-CAY-dee-an) clock that tells our bodies when it’s time to sleep. It’s not a real clock! The circadian clock is a system controlled by neurons, or brain cells, behind your eyes that react to light. This clock tells us to be awake during the day and to go to sleep at night.

Because the human brain evolved before we created electric lights. You’re not reading this on the computer or phone in bed, right? Because that’s a bad idea. The light from the computer tricks your circadian clock into acting like it’s daytime, so you’re not as sleepy. When you stay up late and lose sleep, your brain and body don’t work as well the next day. It’s harder to learn new things, remember old things, and not be cranky. 

For fun further reading, check out MadScieNet's answer to the question, "Do worms sleep?"