Many animals need sleep.
Even brainless jellyfish enter
sleep-like states where they pulse less
and respond more slowly
to food and movement.
But all of the threats and demands
animals face
don’t just go away when it’s time to doze.
That’s why a range of birds and mammals
experience some degree
of asymmetrical sleep where parts
of their brain are asleep
and other areas are more active.
This is even true for humans.
So how does it work?
All vertebrate brains consist
of two hemispheres: the right and left.
Brain activity is usually similar
across both during sleep.
But during asymmetrical sleep,
one brain hemisphere can be in deep sleep
while the other is in lighter sleep.
And in an extreme version
called “unihemispheric sleep,”
one hemisphere may appear completely awake
while the other is in deep sleep.
Take bottlenose dolphins.
Their breathing is consciously controlled,
and they must surface for air every
few minutes or they’ll drown.
When they have a newborn calf, they must
actually swim nonstop for weeks
in order to keep it safe.
So dolphins sleep unihemispherically,
with just one hemisphere at a time.
This allows them to continue swimming
and breathing while snoozing.
Other marine mammals also
need asymmetrical sleep.
Fur seals might spend weeks on end
migrating at sea.
They slip into unihemispheric sleep
while floating horizontally,
holding their nostrils above the surface,
closing their upward-facing eye,
and keeping their
downward-facing eye open.
This may help them stay alert
to threats from the depths.
Similar pressures keep birds
partially awake.
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Mallard ducks sleep in groups, but some
must inevitably be on the peripheries.
Those ducks spend more time
in unihemispheric sleep,
with their outward-facing eyes open
and their corresponding
brain hemispheres more active.
Other birds have been shown to catch z’s
in midair migration.
While undertaking non-stop transoceanic
flights of up to 10 days,
frigatebirds either sleep with one
or both hemispheres at a time.
They do so in seconds-long bursts,
usually while riding air currents.
But the frigatebirds still sleep
less than 8% of what they would on land,
suggesting a great tolerance
for sleep deprivation.
It’s currently unclear whether
asymmetrical sleep
packs the same benefits as sleep
in both hemispheres
and how this varies across species.
In one experiment, fur seals relied
on asymmetrical sleep
while being constantly stimulated.
But in recovery, they showed
a strong preference
for sleep across both hemispheres,
suggesting that it was more restorative
for them.
Dolphins, on the other hand,
have been observed to maintain high levels
of alertness for at least five days.
By switching which hemisphere is awake,
they get several hours of deep sleep
in each hemisphere
throughout a 24-hour period.
This may be why unihemispheric sleep alone
meets their needs.
So, what about humans?
Have you ever woken up groggy after
your first night in a new place?
Part of your brain might’ve spent
the night only somewhat asleep.
For decades, scientists have recognized
that participants sleep poorly
their first night in the lab.
It’s actually customary to toss
out that night’s data.
In 2016, scientists discovered
that this “first night effect”
is a very subtle version
of asymmetrical sleep in humans.
They saw that, during the first night,
participants experience deeper sleep
in their right hemisphere
and lighter sleep in their left.
When exposed to infrequent sounds,
that lighter sleeping left hemisphere
showed greater bumps in activity.
Participants also woke up and responded
to infrequent sounds faster
during the first night than
when experiencing deep sleep
in both hemispheres
during nights following.
This suggests that, like other animals,
humans use asymmetrical sleep
for vigilance,
specifically in unfamiliar environments.
So, while your hotel room is obviously
not trying to eat you
and you’re not going to die
if you don’t continue moving,
your brain is still keeping you alert.
Just in case.