Dear Ellie,

Sleep is one of the most important functions of our body. It helps us feel rested and gives us energy. Getting a good night’s sleep is important for our health. But when we don’t get enough sleep, or haven’t slept in a long time, we feel tired.

Almost every animal on earth needs sleep. Cats like me sleep for around 15 hours a day. I like to take cat naps, so I take a lot of short snoozes throughout the day. Getting plenty of sleep helps make sure I have energy to answer questions from kids like you.

Some animals need even more sleep than I do. Koalas sleep for 18 hours a day. But, some other animals need very little sleep to function. Big animals like elephants only need two hours of sleep each day. Some animals even sleep standing up. Horses and giraffes can sleep on their feet, while other creatures, like you and me, are much more comfortable lying down.

For humans like you, the amount of sleep you need depends on your age. Babies need between 14 and 17 hours of sleep a day, but kids your age need around nine and a half hours of sleep a day. Believe it or not, you will spend about two-thirds of your life sleeping.

So, we know that humans and animals need sleep. But why is it that we get tired? Well, this has to do with what’s going on in our brain. To learn more about what makes us feel sleepy, I asked my friend Christopher Davis, who is a clinical associate professor in the Elson S. Floyd College of Medicine at Washington State University. He studies how the nerves in our bodies help regulate our sleep.

Davis told me tiredness is homeostatic, which means our bodies automatically regulate the process by telling us when to sleep and when to wake up. When our bodies need sleep, especially if we don’t get enough sleep or we stay awake for long periods of time, sleep-like brain signals “creep” into our waking brain signals, he said.

These signals come from several areas in the brain, including a structure called the suprachiasmatic nucleus, which is made up of about 20,000 nerve cells. This nucleus is located in a part of our brain called the hypothalamus. This part of our brain receives signals from our eyes. So, when our eyes see light, our brain gets signals that make us feel awake. But when its dark, the signals make us feel tired.

This is part of a process called the circadian rhythm. This works like a natural clock in our body. This clock also controls the production of melatonin, a hormone that helps tell your body when to sleep and when to wake up. When your body releases more melatonin, you feel more tired. The human body starts releasing melatonin in the evening around bedtime and continues throughout the night. Then, the body slowly stops decreasing melatonin in the morning.

Sleep helps restore our bodies, which is why it’s important to get enough sleep every night. When our bodies are rested, we have enough energy to do well in school, play with our friends, and all of the other fun activities we do every day.

Cadee Christian (and Dr. Universe)


Cadee Christian contributed this article. She is a student in the Edward R. Murrow College of Communication at Washington State University.