1105280859 New Scientist - Blue alert - The dark side
of night light.doc
- 10 May 2011 by David
C. Holzman
- Magazine issue 2811. Subscribe
and save
Light from televisions and phones
could be disrupting our clocks (Image: Ryan McVay/Getty)
Blue
alert: The dark side of night light
From streetlights to smartphones, our world is bathed in artificial light
and we're only just waking up to the profound effect it has on us
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"THESE people aren't really
blind, they are lying." So stated one journal editor when confronted by an
experiment whose results seemed impossible.
The experiment involved clocks. Body clocks. Our internal clocks tend to run a little fast
or slow, so if we are deprived of any clues to what time it is, we soon get out
of sync with the day-night cycle. It used to be thought that our everyday
activities kept our clocks on time, but a series of studies in the 1980s
revealed that light is the key. The clincher came in 1986, when Charles Czeisler showed that light could be used to reset people's
clocks in the same way that one might reset a watch.
The findings helped explain why many
blind people suffer periodic sleep disturbances. Because they cannot detect
light, their body clocks go in and out of sync with the day-night cycle. But Czeisler, of Harvard Medical School, knew that the clocks
of a few blind individuals ran on time. How was this possible?
Czeisler showed that their clocks were also set by light - and that
their eyes were somehow detecting it even though these individuals had no
conscious awareness of light. That suggested that our eyes have special light
receptors that are quite separate from those we see with, and that must have
been overlooked despite centuries of research. "That just blew us
away," he says.
After 20 rejections over five years
and numerous additional tests to rule out other explanations, Czeisler's paper was published in 1995. Other
researchers soon identified the mechanism behind what he had found. We now know
there are specialised light-detecting cells in the
retina whose signals go to the master clock in the brain, rather than to the
visual cortex. In some blind people this system remains unaffected by whatever
caused their blindness, allowing their clocks to stay on time.
These discoveries are turning out to
have profound implications. It is becoming clear that even dim lights can
affect our body clocks, meaning simply having the lights on late at night or
staring at a computer screen can disrupt our internal rhythms. What's more, it
turns out that blue light has the greatest power to change our clocks, and
modern lighting is getting bluer. The potential effects go far beyond the
unpleasant, jet-lagged feeling that body-clock disruption can cause. There is
growing evidence that continual disruption is linked in the long term to serious
illnesses including cancer, heart disease and diabetes. It can even alter the
wiring of our brains.
It is not all bad news. Bright light
during the day has, of course, long been known to mitigate the depressive
effect of long dark winters on people who suffer from seasonal affective
disorder, and recent research has demonstrated more general benefits. For
example, elderly nursing-home residents exposed to very bright indoor light
(around 1000 lux - roughly equivalent to outdoor light on an overcast day) for
an hour in the morning were less likely to show signs of depression, according
to a 2008 study (Journal of the American Medical Association, vol 299, p 2642).
Part of the reason for this is that
our central clocks control levels of the hormone melatonin. When it gets dark,
our melatonin levels rise, making us sleepy, while bright light turns off
melatonin production and makes us more alert.
So light at night actually has two distinct effects. It can reset our internal clocks,
as Czeisler showed, and it can also suppress the
production of melatonin. The first to suspect the suppression of melatonin
could affect our health was Richard Stevens at the University of Connecticut Health Center in Farmington. During the
1980s, he was investigating the causes of breast cancer, rates of which are
much higher in developed countries. Stevens came across studies that suggested
that too much light could alter the development of breast tissue and suppress
melatonin secretion, and that lower melatonin might boost oestrogen
levels.
That all came together for him one
night as a street light shone into his apartment. He realised
that the introduction of bright artificial lighting was a profound change in
our environment, one that could be affecting our health in many ways. The idea
became known as the light-at-night hypothesis, and there is growing evidence in
support of it.
Several epidemiological studies suggest
there is indeed a link between light-at-night and cancer, particularly breast
cancer. Perhaps the most direct evidence comes from a study by David Blask of Tulane University School of Medicine, New Orleans,
and collaborators. They implanted human breast tumours
into female rats and pumped the tumours with blood
from healthy women. The blood had been collected either in daylight, or at
night after the women experienced 2 hours of complete darkness, or at night
following 90 minutes under bright fluorescent light.
The melatonin-rich blood taken from
subjects in total darkness severely slowed the tumours'
growth, they found. Conversely, tumours grew much
faster after receiving melatonin-depleted blood from women exposed to light (Cancer
Research, vol 65, p 22274). "We can
manipulate light and melatonin levels, and thus cancer growth rates, almost
like a dimmer switch," says Blask.
Tumours also grow faster in mice made to follow schedules mimicking
shift work or jet lag, says Steven Lockley of Harvard Medical School in Boston.
The evidence implicating shift work in breast cancer is so extensive that in
2007 the World Health Organization categorised shift
work as a probable cause of cancer.
If melatonin is the key, it is
plausible that anything that suppresses melatonin could increase the risk of
cancer. Lockley points out that totally blind women - with no functioning light
receptors at all in their eyes - have a breast cancer risk half that of their
sighted counterparts. "The totally blind women never have their melatonin
perturbed, which may be the reason why their cancer risk is less," he
says.
Besides cancer, disruption
of our body clock and melatonin suppression have been linked to obesity,
diabetes and cardiovascular disease. Studies show that night-shift workers have
higher rates of heart attack and stroke than those on day schedules, for
instance, and that the difference grows with the number of years spent doing
the job.
Impaired
thinking
Animal studies show that disrupted
routines can even alter the wiring of the brain, impairing cognitive function, it was reported earlier this year (Proceedings of the National Academy of Sciences, vol 108, p 1657). Ilia Karatsoreos
of The Rockefeller University in New York found that mice kept on an unnatural
cycle of 10 hours of light followed by 10 hours of darkness lost neuronal
complexity in the prelimbic prefrontal cortex, an
executive part of the brain. Karatsoreos thinks the
results are relevant to people. "I think this study is proof in principle
that disrupting the clock by changing the light cycle can result in changes in
the brain, behaviour and physiology," he says.
However, imposing a 20-hour cycle is
like "hitting the system over the head with a hammer", he cautions.
It remains to be seen if milder disruptions also have these effects.
Meanwhile, studies have been showing
that the blue wavelengths are by far the most powerful in shifting rhythms and
suppressing melatonin. In 2001, George Brainard of
Thomas Jefferson University in Philadelphia, Pennsylvania, and collaborators
found that melatonin secretion was most powerfully suppressed when volunteers
were exposed to very bright light at around 2 am, at wavelengths from 450 to
480 nanometres - squarely in the blue part of the
spectrum ( Journal of Neuroscience, vol
21, p 6405).
The findings suggested that the
special receptor cells in our retinas contain a light-sensitive protein
distinct from those we see with, and that it responds mainly to blue light.
Sure enough, the cells were shown to contain a protein called melanopsin the following year.
In similar experiments involving
extended nocturnal exposure to light, Brainard, Czeisler and Lockley showed that pure blue light of 460 nm
suppressed melatonin for roughly twice as long as green light of 555 nm (Journal of
Clinical Endocrinology & Metabolism, vol 88,
p 4502). The blue light also reset people's internal clocks by 3 hours on
average, compared with just an hour and a half for green light. Resetting
clocks in this way means people find it hard to get to sleep the following
night, and then feel tired in the morning.
More evidence comes from a study led
by Leonid Kayumov at the University of Toronto, Canada. He asked some volunteers to wear goggles
designed to filter out blue light. When volunteers did simulated shift work
under bright indoor light (800 lux), melatonin production was suppressed in
those not using the goggles, whereas those wearing goggles had melatonin
secretion profiles similar to those of subjects exposed to dim light (Journal of
Clinical Endocrinology & Metabolism, vol 90,
p 2755). This suggests the use of such goggles could minimise
the impact on shift workers or people staying up late (see
"Use light right").
While blue light is worst in terms
of affecting our body clocks at night, it is also the best kind of light to
have by day. Dieter Kunz of the Clinical Chronobiology Research Group at Charitι University of Medicine in Berlin, Germany, waxes
lyrical about the benefits of blue. "Bright blue in the morning is
incredible. Throw away the pills," he jokes. Blue light also has the
greatest power to keep us alert. Lockley has shown that people exposed to pure
blue light responded faster in tests and made fewer mistakes than those exposed
to pure green light (Sleep, vol 29, p 161).
So blue
wavelengths appear to have the greatest influence on human physiology, day or
night. There have been claims that full-spectrum
lighting, which contains a mixture of all visible wavelengths and resembles
natural daylight, is best for working environments, but the level of blue
matters most as far as alertness is concerned.
These findings suggest that if light
at night is a serious issue, it could be getting worse. Low-energy fluorescent
bulbs and LED-based lighting usually produce much more blue light than the
old-fashioned tungsten light bulbs they are replacing (see "True colours").
What's more, while most studies into
the effects of night-time light have involved intense illumination over
extended periods, recent studies are showing that normal home lighting and even
dim light may be disruptive to human physiology. A study published earlier this
year, for instance, found that for people exposed to normal room lighting in
the late evening - less than 200 lux - melatonin levels rose later than in
people subjected to dim lighting, and then remained high for about 90 minutes
less (Journal
of Clinical Endocrinology & Metabolism, vol
96, p 463). "One hundred lux gives 50 per cent of the maximal response
under very bright light, and melatonin suppression can be measured at much
lower light levels," says Lockley.
Besides suppressing melatonin, even
relatively dim light sources such as table lamps and computer monitors can set
back our internal clocks. "Our lab has shown that less than 8 lux is
capable of entraining the circadian clock," says Lockely.
The team speculates that this might explain the high prevalence of delayed
sleep phase disorder, in which people have trouble getting to sleep and then
wake up feeling tired. So how serious is this problem? "We have no idea
what chronic low-light exposure does as the entire world is self-experimenting
on using electric light at night," he says.
The degree of harm is likely to
depend on the degree of disruption, Lockley says, but it would take a very
large study to prove this. However, there is already plenty of evidence linking
short sleep duration to increased risks of cardiovascular disease, stroke, high
blood presssure, diabetes and depression.
It is a problem people can do
something about. While researchers remain reluctant to provide specific
guidelines for night-time lighting, we can get a glimpse of the latest thinking
in this area from NASA. It recently reduced the upper limit of illumination in
the general sleeping areas of spacecraft, where some astronauts might be active
as their colleagues doze, from 20 lux to 1 lux (a lux
is roughly equivalent to full moonlight). For dedicated sleeping areas, the
upper limit is 0.02 lux (equivalent to a quarter moon).
Manufacturers could also help by
selling lights for use at night that produce less blue. In fact, one newly launched kind of low-energy lighting, called
ESL, has a spectrum more like that of incandescent
bulbs.
Changing light bulbs is relatively easy.
The hard part will be persuading people to turn off their TVs and put down
their iPads well before they go to sleep.
Use
light right
Be Alert in the day, sleep well at
night
Get lots of bright light during
the day, especially in the morning. It will make you more alert and happier,
and help you sleep at night.
As you get older, you will need
more light during the day. The lens of the eye lets less light through as you age: in
particular, it lets through less blue light, which is most important for
setting your clock.
Dim the lights well before your
bedtime. That means no bright screens, either - including televisions,
computers and smartphones.
Maintain a consistent bedtime and
wake time from day to day.
Time spent in the dark makes your
body clock more sensitive to light. If you have to get up during the night, use
a dim red light to minimise any disruption.
Avoid caffeine late in the day and
develop a relaxing bedtime routine.
If, despite doing all the above,
you still struggle to sleep, try wearing amber-coloured goggles in the hours before bed. They are commercially
available and designed to filter out blue wavelengths.
David C. Holzman is a freelance science writer based in Lexington,
Massachusetts
- From issue 2811 of New Scientist
magazine, page 44-47.
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