How does your body know what time it is? - Marco A. Sotomayor

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Being able to sense time helps us do everything from waking and sleeping to knowing precisely when to catch a ball that’s hurtling towards us. And we owe all these abilities to an interconnected system of timekeepers in our brains. But how do they work? Marco A. Sotomayor details how human bodies naturally tell time.

Lesson by Marco A. Sotomayor, animation by TOGETHER.

In 1962, a cave explorer
named Michel Siffre

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

without light or clocks.

He attached himself to electrodes
that monitored his vital signs

and kept track of when he slept and ate.

When Siffre finally emerged,

the results of his pioneering experiments

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

Despite having no external cues,

he fell asleep,

woke up,

and ate at fixed intervals.

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

Scientists later found these rhythms
affect our hormone secretion,

how our bodies process food,

and even the effects
of drugs on our bodies.

The field of sciences studying
these changes is called chronobiology.

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

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

We owe all these abilities to
an interconnected system of timekeepers

in our brains.

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

a clock counting the hours of the day,

and a calendar notifying us
of the seasons.

Each one is located in
a different brain region.

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

in the suprachiasmatic nucleus, or SCN
of the hypothalamus.

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

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

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

they make another protein called PER.

When enough PER accumulates,

it deactivates the gene that makes CLK,

eventually making us fall asleep.

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

allowing CLK to rise,

starting the cycle over.

There are other proteins involved,

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

between CLK by day and PER by night.

For more precision,

our SCNs also rely
on external cues

like light,

food,

noise,

and temperature.

We called these zeitgebers,

German for "givers of time."

Siffre lacked many
of these cues underground,

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

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

it helps wake us up.

Traveling through the optic nerve
to the SCN,

it communicates what's happening
in the outside world.

The hypothalamus then halts
the production of melatonin,

a hormone that triggers sleep.

At the same time,

it increases the production
of vasopressin

and noradrenaline throughout the brain,

which help control our sleep cycles.

At about 10 am,

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

and later in the afternoon,

it also improves our muscle activity
and coordination.

Bright screens at night can confuse
these signals,

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

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

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

One theory for how this works
involves the fact

that communication between a given
pair of neurons

always takes roughly the same
amount of time.

So neurons in our cortex
and other brain areas

may communicate in scheduled,
predictable loops

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

That creates our perception of time.

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

Every day, he challenged himself
to count up to 120

at the rate of one digit per second.

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

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

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

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

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

Could each of us
be experiencing it differently?

Only time will tell.