New Scientist - USA (2021-02-27)

54 | New Scientist | 27 February 2021

Dropping off

How do our brains stop us from

falling out of  bed while asleep?

Michael Barry Henderson

Maroochydore,

Queensland, Australia

I occasionally fall out of bed while

asleep, and it hurts because I fall

onto a ceramic floor. I am a toss-

and-turn-in-bed person and move

from one side to the other. Every

now and then, it just so happens

that I go over the edge.

I don’t think it is a matter

of your brain’s control of your

body, just your bed-parking.

Michael Bennett

Lowood, Queensland, Australia

I am not sure that our brains do

prevent us from falling out of bed.

Recently, I dreamed that I was

washed off a cliff by a large ocean

wave. This must have caused me

to fall out of bed because I woke

up on the floor. I didn’t have any

history of falling out of bed or

sleepwalking before this incident.

The question is: how could I

have been physically transported

by an external force (the wave)

that I perceived to be outside

my control, acting through my

subconscious mind in a dream?

This seems to be rather different

from sleepwalking, in which the

subconscious directs you to move.

Incidentally, many years ago, I was

washed over by a similar wave

while rock climbing, but I was

securely attached to the rock

and was therefore immobile.

Larry Curley

Huntingdon, Cambridgeshire, UK

They don’t. When I was a young

lad, I would often fall out of bed

when I was asleep. On hearing a

thud, my mother would enter the

Want to send us a question or answer?

Email us at [email protected]

Questions should be about everyday science phenomena

Full terms and conditions at newscientist.com/lw-terms

The back pages Almost the last word

ED

EL

MA

NN

/SC

IEN

CE

PH

OT

O^ L

IBR

AR

Y

bedroom and sometimes

find me asleep on the floor.

David Muir

Edinburgh, UK

Rapid eye movement (REM)

sleep is a recurring part of the

sleep cycle and is associated

with dreaming.

We are prevented from

physically acting out our dreams

by REM atonia, a partial paralysis

of the body that is caused by the

inhibition of motor neurons,

the nerves that control muscles.

This is brought about by

neurotransmitters that

affect the brainstem.

There are people who lack this

inhibition and act out dreams

with vocalisations and sudden

arm and leg movements. This is

called REM sleep behaviour

disorder. People with this

condition can injure themselves

or their bed-partners, and they are

also more likely to fall out of bed.

A more prosaic way of thinking

about the origins of REM atonia is

to consider our ancient ancestors,

who, a few million years ago,

probably built sleeping platforms

in trees for their protection.

Anyone who lacked the genes that

control REM atonia would be more

likely to fall from their platforms

and be injured or despatched

by ground-roaming predators.

Their genome would

therefore have been removed

from the future hominin gene

pool, resulting in people today

being less likely to land on the

floor during our slumbers.

Myopic wildlife

Many people are long or

short-sighted. Is this the same

for other animals, and if so,

how do they cope?

Eleanor Caves

University of Exeter, Devon, UK

There aren’t many studies of

near and far-sightedness in

non-human animals, and

those that do exist are mainly

restricted to primates.

However, different species of

animals vary a great deal in their

ability to perceive fine detail. This

“visual acuity” can be measured

in units called cycles per degree,

which measure how much detail

can be discerned in a given scene.

Humans have a visual acuity

of around 60 cycles per degree,

whereas for house cats, it is

10 cycles per degree. Many insects

have an acuity of around 1 cycle

per degree or less.

In general, visual acuity is

correlated with eye size, so species

with larger eyes – which also tend

to have larger body sizes – have a

higher acuity. This holds within

species too. In my work on a

freshwater fish called the green

swordtail, I have found that

females can have an acuity

ranging from 1 to 5 cycles per

degree, depending on their size.

In some insect species, males

and females differ in their visual

acuity. Males sometimes even

have areas of their eyes with

increased acuity that they use to

spot fast-flying potential mates.

As for how animals cope

with the limitations of their

eyesight, there may be several

answers. Although they may

not be able to perceive fine

spatial detail, this might not

be detrimental to their lifestyle,

so they might not need to cope.

A shrimp that lives full-time on an

anemone and feeds by scavenging,

for example, might not require

high visual acuity anyway.

This week’s new questions

Rowdy roads Why is the noise from traffic on roads

louder when it is raining or the road is wet than it is

on a dry day? Kate Macdonald, Bath, UK

Cutting the cord Why are the umbilical cords of human

infants tied or cut, when the young of other animals

don’t need this intervention? Patricia Hodges,

Bromsgrove, Worcestershire, UK

Why do we cut or tie

the umbilcal cord of

a newborn baby?

“ I dreamed that a large

ocean wave washed

me off a cliff. This

must have caused me

to fall out of bed, as I

woke up on the floor”