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In 1962, a cave explorer 
named Michel Siffre

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started a series of experiments where
he isolated himself underground for months

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without light or clocks.

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He attached himself to electrodes
that monitored his vital signs

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and kept track of when he slept and ate.

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When Siffre finally emerged,

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the results of his pioneering experiments

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revealed that his body had kept
to a regular sleeping-waking cycle.

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Despite having no external cues,

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he fell asleep,

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woke up,

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and ate at fixed intervals.

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This became known as a circadian rhythm
from the Latin for "about a day."

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Scientists later found these rhythms
affect our hormone secretion,

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how our bodies process food,

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and even the effects 
of drugs on our bodies.

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The field of sciences studying
these changes is called chronobiology.

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Being able to sense time helps us do
everything from waking and sleeping

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to knowing precisely when to catch a ball
that's hurtling towards us.

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We owe all these abilities to 
an interconnected system of timekeepers

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in our brains.

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It contains the equivalent of a stopwatch
telling us how many seconds elapsed,

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a clock counting the hours of the day,

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and a calendar notifying us 
of the seasons.

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Each one is located in 
a different brain region.

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Siffre, stuck in his dark cave, relied
on the most primitive clock

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in the suprachiasmatic nucleus, or SCN
of the hypothalamus.

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Here's the basics of how we think it works
based on fruitfly and mouse studies.

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Proteins known as CLK, or clock,
accumulate in the SCN throughout the day.

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In addition to activating genes
that tell us to stay awake,

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they make another protein called PER.

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When enough PER accumulates,

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it deactivates the gene that makes CLK,

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eventually making us fall asleep.

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Then, clock falls low, so PER 
concentrations also drop again,

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allowing CLK to rise,

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starting the cycle over.

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There are other proteins involved,

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but our day and night cycle may be driven
in part by this seesaw effect

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between CLK by day and PER by night.

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For more precision,

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our SCNs also rely 
on external cues

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like light,

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food,

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noise,

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and temperature.

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We called these zeitgebers,

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German for "givers of time."

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Siffre lacked many 
of these cues underground,

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but in normal life, they fine tune
our daily behavior.

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For instance, as natural morning light
filters into our eyes,

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it helps wake us up.

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Traveling through the optic nerve
to the SCN,

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it communicates what's happening
in the outside world.

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The hypothalamus then halts
the production of melatonin,

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a hormone that triggers sleep.

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At the same time,

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it increases the production
of vasopressin

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and noradrenaline throughout the brain,

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which help control our sleep cycles.

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At about 10 am,

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the body's rising temperature drives up
our energy and alertness,

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and later in the afternoon,

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it also improves our muscle activity
and coordination.

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Bright screens at night can confuse
these signals,

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which is why binging on TV before bed
makes it harder to sleep.

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But sometimes we need to be
even more precise when telling the time,

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which is where the brain's internal 
stopwatch chimes in.

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One theory for how this works 
involves the fact

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that communication between a given
pair of neurons

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always takes roughly the same
amount of time.

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So neurons in our cortex 
and other brain areas

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may communicate in scheduled,
predictable loops

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that the cortex uses to judge
with precision how much time has passed.

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That creates our perception of time.

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In his cave, Siffre made a fascinating
additional discovery about this.

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Every day, he challenged himself 
to count up to 120

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at the rate of one digit per second.

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Over time, instead of taking two minutes,
it began taking him as long as five.

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Life in the lonely, dark cave had warped
Siffre's own perception of time

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despite his brain's best efforts
to keep him on track.

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This makes us wonder what else influences
our sense of time.

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And if time isn't objective, 
what does that mean?

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Could each of us 
be experiencing it differently?

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Only time will tell.