2019-06-01_New_Scientist

34 | New Scientist |1 June 2019

M

ORTEN HALMØ PETERSEN used to

live in a windowless basement flat

in Copenhagen. If he didn’t get out

in the daytime, he would lose track of time and

start becoming irritated and depressed. “When

you are living in a basement with only artificial

light, it becomes very clear that something is

lacking,” he says. “It’s an emotional, physical

and mental thing all combined.”

You can say that again. Our lifestyles have

rapidly changed our relationship with light.

Prior to the invention of gas lighting at the

turn of the 19th century, the only artificial light

we could rely on was from flickering firelight,

candles or whale-oil lamps. People also spent

many more of their waking hours outside.

Today, the average Westerner spends

90 per cent of their life indoors. That means we

are getting less light during the day and being

exposed to more light at night. This pattern is

increasingly being linked to disrupted sleep

and circadian rhythms – 24-hour fluctuations

in our biology and behaviour – with

consequences for our physical and mental

health. Meanwhile, getting too little sunlight

is contributing to vitamin D deficiency and

may be undermining our immune and

cardiovascular systems as well.

Our changed relationship with the sun is

profoundly affecting our biology. That’s why

people like Petersen are being recruited by

researchers to help investigate how much

damage we do by shying away from the light,

and just how much light we need. The good

news is researchers are finding that even

small increases in your exposure to bright

light during the day have a wide range of

benefits, from improving sleep and mood to

speeding recovery from serious illness.

Today, most of us spend our daytimes in

the equivalent of twilight. Illuminance is

measured in lux, which refers to the amount

of light striking a surface (see “The light in our

lives”, page 36). Studies of the Amish, who lead

an off-grid and more agrarian existence,

highlight how much our relationship with

light has changed. In summer, the Amish are

Many of us spend too much time in

a twilight of artificial illumination. We skimp on

sunshine at our peril, finds Linda Geddes

exposed to an average daytime illuminance

of 4000 lux, compared with 587 lux for the

average person in the UK. In winter, that drops

to just 210 lux in the UK, compared with 1500

lux for the Amish. During the evening, the

average illuminance in Amish homes is

around 10 lux – up to five times lower than

evening light levels in electrified homes.

At the back of the eye, behind the rod and

cone cells that enable our brains to construct

images, are light-responsive cells called

intrinsically photosensitive retinal ganglion

cells (ipRGCs). These fire in response to any

light if it is bright or sustained enough but

they are particularly sensitive to light in the

blue part of the spectrum. This includes bright

daylight, many LEDs and light from screens.

These ipRGCs send signals to areas of the

brain that control alertness. One study found

that exposure to an hour of low-intensity blue

light increased people’s reaction speeds more

than if they had drunk two cups of coffee. The

cells also send signals to a tiny patch of brain

tissue called the suprachiasmatic nucleus

(SCN). This functions as the body’s master

clock, tweaking the timing of the circadian

clocks ticking in every cell of our bodies to

keep them synchronised with each other and

with the external time of day.

Exposure to light in the evening delays the

daylight

Living

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