How do we know?
↻ One of the pages with a moving picture
Researchers watched fluid flush through sleeping mouse brains.
- Your brain does not switch off. It stays remarkably active while you sleep, and sleep helps you learn and form long-term memories.
- In kids and teens, sleep also helps support growth and development.
- Scientists think sleep may help clean the brain. In mice, a fluid-flushing system was almost 10 times more active during sleep.
- Most dreaming happens during REM sleep, and the brain sends signals to relax our muscles so we don’t act out our dreams.
Researchers led by Maiken Nedergaard watched fluid flush through the brains of mice. The flushing was much stronger when the mice slept: a fluid-flushing system was almost 10 times more active. Scientists are still testing how this works in people.
In 2013, scientists in Rochester, New York, reported that sleeping mouse brains “take out the trash”.
Kids 5 to 12 years old need about 9 to 12 hours of sleep each night.
Wow number
9–12 Kids 5 to 12 years old need about 9 to 12 hours of sleep each night!
Myth or fact?
When you fall asleep, your brain switches off. Myth. Your brain stays remarkably active while you sleep.
People spend about one-third of their time asleep. Fact. That’s about 8 hours out of every 24.
Think about it
Everyone needs sleep, but scientists still call its full purpose a mystery. Why do you think animals sleep?
Where in the world?
Rochester, New York, USA where, in 2013, scientists reported that sleeping mouse brains “take out the trash” — 43.1566° N, 77.6088° W.
Where these facts come from
Every number on page 73, and who measured it
This page tells us how much sleep kids need each night.
How Much Sleep Do Kids Need? — KidsHealth, a health site for kids from Nemours Children's Health
kidshealth.org/en/kids/not-tired.html
We checked it on 31 October 2026
This page tells us how sleep helps kids learn and grow.
How Sleep Works - Why Is Sleep Important? — The U.S. National Heart, Lung, and Blood Institute
nhlbi.nih.gov/health/sleep/why-sleep-important
We checked it on 31 October 2026
This page tells us that the brain stays busy while we sleep.
Brain Basics: Understanding Sleep — The U.S. National Institute of Neurological Disorders and Stroke, brain scientists
ninds.nih.gov/health-information/public-education/brain-basics/brain-basics-understanding-sleep
We checked it on 31 October 2026
This page tells us about the Rochester study of the brain cleaning itself during sleep in mice.
To Sleep, Perchance to Clean — The University of Rochester Medical Center in New York
urmc.rochester.edu/news/story/to-sleep-perchance-to-clean
We checked it on 31 October 2026
Has this changed?
Yes, this one may change. Scientists still call the full purpose of sleep a mystery, and they are still testing how the brain-cleaning idea works in people.
Sleep helping you learn and grow is well understood.
Words from this page
- long-term memory
- Something your brain keeps for a long time.
- REM sleep
- The kind of sleep when most dreaming happens.
Try it!
Keep a sleep diary.
1. For one week, write down what time you go to bed and wake up. 2. Each morning, rate how you feel: sleepy, OK or great. 3. Add up your hours. Do you feel better after more sleep?
What happens: you can spot patterns, like how your mood matches the hours you slept.
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