In the early 1950s, graduate student Eugene Aserinsky was monitoring sleeping subjects at the University of Chicago. He was using an old Offner Dynograph, a machine that recorded brainwaves and eye movements. Suddenly, the machine’s pens began swinging so wildly he thought it must be broken. But the machine was fine. His sleeping subject’s eyes were moving rapidly. Aserinsky and his mentor, professor Nathaniel Kleitman, had discovered a strange new phase of sleep. They called it Rapid Eye Movement, or REM, publishing their findings in the journal Science on September 4, 1953. When they woke people during these periods, they recalled vivid, visual dreams. The discovery proved the brain does not just shut off at night; it enters a different, highly active world of its own.
Why We Sleep
Science reveals why we sleep, from the discovery of active brain stages to the nightly clearance of toxic waste from between our brain cells.
The Broken Machine
Part of
Frequently Asked Questions
Is the brain 'off' when we sleep?
No, sleep is a highly active state. The 1953 discovery of REM sleep showed the brain cycles through complex stages, with heart rate increasing by 10 percent and respiration by 20 percent during dreaming, challenging the old idea that it simply shuts down.
What actually makes us feel tired?
A chemical called adenosine builds up in the brain the longer you are awake, creating 'sleep pressure'. This signal works alongside your internal 24-hour clock, or circadian rhythm, which manages alertness based on light, to regulate when you feel sleepy.
Does sleep really clean the brain?
Yes. During deep sleep, a network called the glymphatic system becomes ten times more active than when you are awake. The space between brain cells expands by 60%, allowing cerebrospinal fluid to flush out toxic waste products that accumulate during the day.
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