illuminance. Research in mice may provide
insight into why this happens. When mice
were injured to simulate a heart attack, there
was a significant difference in the number and
type of immune cells that rallied to the heart,
and in the amount of scar tissue that formed,
depending on whether they recovered in a
cage with a disrupted light environment, as
you would find in many hospitals, or a more
natural 24-hour, light-dark cycle. Those with
disrupted rhythms were more likely to die
from their injury.
As for whether human-centric lighting can
help those who aren’t living in a care home or
recovering in hospital, there is some evidence
that it can improve healthy people’s sleep.
But Figueiro is less convinced by other claims,
for example that it boosts productivity. “In
many cases, human-centric lighting is being
oversold and overstated,” she says. “Usually,
people are only talking about spectrum when
it comes to circadian lighting. They are not
talking about the amount, timing or
distribution, and it is how much light that’s
getting into the eyes that matters.”
There is still much to learn about how
light – and darkness – affect our biology. “How
much light do we actually need, at what time
of day do we need what, and how often do we
need it? We just don’t know these things at the
moment,” says Wulff. Yet as a basic principle,
we should all be striving to brighten our days
and darken our nights. We evolved on a planet
with a 24-hour cycle of darkness and light.
It is time to reconnect with those extremes. ❚You have probably heard that
being exposed to blue light in
the evening can disrupt your
circadian rhythms and imperil
your health. But how bright – and
how blue? Mariana Figueiro and
her colleagues of the Lighting
Research Center at Rensselaer
Polytechnic Institute in New
York state are testing this by
measuring melatonin, a hormone
linked to sleep that rises during
the evening in response to a
signal from your body clock.
Several studies have
suggested that the light from
smartphones can disrupt sleep,
but Figueiro’s team found that
only larger devices produce
enough bluish-white light to
affect levels of melatonin: for
adults, it was 85 lux for 1 hour’s
exposure, while for adolescents
it was 71 lux. “A larger iPad could
provide that amount of light,
or a little bit more, but phones
typically don’t,” says Figueiro.
However, they found that lower
light intensities could suppress
melatonin if people were
exposed to them for longer
periods of time.
For light sources emitting
a warmer colour, such as
incandescent light bulbs or
warm-white LEDs, the typical
levels found in a living room in
the evening wouldn’t suppress
melatonin production. Nor
would a TV screen watched
from 2 metres or more.
That may be key: how close
the light is to your eyes can make
a difference. In a separate study,
Figueiro discovered that even in
night-shift mode, which reduces
light from the blue part of the
spectrum, an iPad could suppress
melatonin. Her recommendation:
“Just turn it off.”was left unchanged. After 3.5 years, residents
who were exposed to the brighter daytime
lighting showed less cognitive deterioration
and fewer symptoms of depression. When
bright light was combined with melatonin
supplements, they also slept better and
were less agitated.Bright ideas
Others have tried to replicate these findings,
with mixed results. Figueiro suspects that
may be down to inconsistencies in the amount
of light actually being delivered into each
person’s eyes. “If people don’t take their
medication, you’re not going to see any effect,
but that doesn’t mean the medication doesn’t
work,” she says. “It is the same thing with
light.” To address this, she has been trialling
a “light table” that directs light upwards into
people’s eyes. A soon-to-be-published study
will show that this improves sleep, reduces
depression and decreases agitation in people
with dementia, she says.
Poor lighting is also a problem in many
hospitals, which tend to have dim lights left
on day and night. A 2017 study found that the
average daytime illuminance in a UK intensive
care unit was 159 lux. At night, it was 10.
People being cared for in such units often
have disrupted circadian rhythms, which
are associated with impaired recovery from
illness, says David Ray at the University of
Manchester, UK. Compounding the problem,
certain drugs, including morphine,
can further disrupt circadian clocks.
Hospital patients recover faster when
they have greater access to daylight. In one
study, the average length of stay for people
recovering from heart surgery was reduced by
7.3 hours for every 100 lux increase in daytimeLinda Geddes is a New Scientist
consultant and author of Chasing the
Sun: The new science of sunlight and
how it shapes our bodies and mindsJUST HOW BAD
IS BLUE LIGHT?In the West
people spend
almost all their
time indoors,
in artificial light38 | New Scientist |1 June 2019
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