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Sleep: fun facts and quiz
The essentials in 7 ideas, with fun facts, an illustrated story and a quiz to see how much you know.
The essentials
Why We Sleep
Sleep lets the body repair itself and the brain sort memories. Adults generally need 7 to 9 hours a night; teenagers need more.
Key fact
We spend about a third of our lives asleep.
Sleep Cycles
Sleep runs in cycles of about 90 minutes, moving from light sleep to deep sleep to REM sleep, when most vivid dreams happen.
Key fact
REM stands for Rapid Eye Movement.
Your Body Clock
Your body clock runs on a roughly 24-hour cycle and tells you when to feel sleepy. Daylight resets it, and evening darkness triggers the release of melatonin, the sleep hormone.
Key fact
Jet lag happens when your body clock is out of step with local time.
How Animals Sleep
Animals sleep in amazing ways. Dolphins rest one half of the brain at a time, giraffes may sleep under 2 hours a day, and brown bats sleep about 20.
Key fact
Some birds can sleep while flying.
Dreams
Everyone dreams, even people who don’t remember it. Dreams may help process emotions and memories, but scientists still debate exactly why we have them.
Key fact
You’re more likely to remember a dream if you wake up during it.
Screens & Sleep
Bright screens late at night can delay sleepiness, partly by holding back melatonin. A calm, dark, cool bedroom helps most people sleep better.
Key fact
A cool room, around 18 °C, suits sleep for many people.
Sleep Debt
Missing sleep adds up as “sleep debt”. Tired people react more slowly: staying awake for about 17 to 19 hours can impair you as much as a small amount of alcohol.
Key fact
A short nap of 10 to 20 minutes can boost alertness.
Fun facts
Even jellyfish, which have no brain, show signs of sleep, so sleep may be hundreds of millions of years old.
During REM sleep most of your muscles are paralysed, which stops you acting out your dreams.
Your body temperature follows your body clock too: it’s lowest in the early hours of the morning.
Sea otters often hold paws while they sleep on the water, so they don’t drift apart.
People blind from birth dream in sounds, smells and touch rather than pictures.
Before electric light, many people seem to have slept in two stretches, with a waking hour in the middle of the night.
Very tired people can have “microsleeps”, lapses of a few seconds, even with their eyes open.
Story: A Month in the Dark
A Month in the Dark
1938. Sleep scientist Nathaniel Kleitman and a student move into Kentucky’s Mammoth Cave for 32 days, far from any daylight.
They try to live on a 28-hour day. The young student adjusts. Kleitman never does, and still gets sleepy at his old times.
Our body clocks don’t reset easily. Kleitman later helped discover REM sleep, in 1953.
True or false?
Tap your answer to see if you got it, and why.
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Question 1 of 10
Sleep helps the brain store memories.
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True. It sorts and strengthens what you learned.
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Question 2 of 10
Most vivid dreams happen during REM sleep.
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True. Your eyes dart around under closed lids.
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Question 3 of 10
Daylight helps reset your body clock.
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True. Morning light is especially powerful.
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Question 4 of 10
Dolphins can sleep with half their brain at a time.
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True. The other half keeps them breathing and alert.
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Question 5 of 10
People who don’t remember dreams still dream.
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True. They just forget them on waking.
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Question 6 of 10
Bright light at night can delay sleepiness.
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True. It tells your body clock it’s still day.
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Question 7 of 10
Lack of sleep slows your reactions.
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True. Tiredness dulls attention and reaction time.
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Question 8 of 10
During sleep, the brain switches off completely.
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False. It’s busy sorting and repairing.
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Question 9 of 10
A sleep cycle lasts about 10 minutes.
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False. About 90 minutes.
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Question 10 of 10
Melatonin makes you feel more awake.
Show the answer
False. It helps you feel sleepy.
Keep going in the app
KnowSoda learns what you know and brings back what you’re about to forget. Sleep has 6 more ideas at the Advanced and Expert levels.
Sources
- NHS — Sleep and tiredness
- U.S. National Institutes of Health — Brain basics: understanding sleep
- Sleep Foundation — Sleep cycles, circadian rhythm
- Williamson & Feyer (2000), sleep deprivation and alcohol intoxication
- Rattenborg et al. (2016), evidence that birds sleep in flight, Nature Communications
- Xie et al. (2013), sleep drives metabolite clearance from the adult brain, Science 342
- Konkoly et al. (2021), real-time dialogue with dreamers, Current Biology 31
