Night vision decreases with age
March 27, 2007

How well do you see at night? If you're over 50, probably not as
well as you think, no matter how many carrots you eat. The typical
50-year-old driver needs twice as much light to see as well after
dark as a 30-year-old. Yet few of us compensate adequately for the
reduction in nighttime acuity that occurs in the aging eye.
Changes in driving habits are crucial, and so are adjustments at
home to prevent the all-too-common accidents that land older folks
in the hospital.
In a normal healthy eye, light enters through the pupil and passes
through the lens, which focuses it and directs it to the retina on
the back of the eye, where images form.
The retina contains two kinds of photoreceptors: cones and rods.
Cones enable us to see when it is light. They give us color vision
and allow us to see details like the words on this page. Rods are
very sensitive, especially to motion. They provide only
black-and-white images and thus are critically important for night
vision.
The human eye changes gradually with age, but the changes are
critical, as the Harvard Health Letter described in its March 2006
issue.
In dim light or darkness, eyes adapt by widening the pupils to let
in as much light as possible. The iris (the colored part of the eye
surrounding the pupil) contains tiny muscles that control the size
of the pupil. As you get older, these muscles weaken and do not
respond as well to the need to let in more light. The result is a
small pupil when you try to see in poor light. It's as if your eyes
were still young but you were wearing sunglasses at night.
There is also evidence that as we age we lose more rods than cones.
In the young eye, rods outnumber cones by 9 to 1 in the part of the
retina called the macula. But an autopsy study of older adults found
that while the cones remained intact, almost a third of the rods in
the macula had been lost.
The less responsive muscles in the iris also affect the eye's
ability to adjust when the intensity of light changes, such as when
a car with its headlights on approaches and then passes.
Another common change in older eyes is a gradual clouding of the
lens - the formation of cataracts - which makes the lens less
transparent and reduces the amount of light reaching the retina.
Cloudy lenses also scatter light. This can cause temporarily
blinding glare from, for example, the headlights of an approaching
vehicle at night.
Traffic deaths are 3 times greater at night than during the day,
though only 20 percent of driving is done after dark. Fatigue and
alcohol are 2 important causes, but experts say the biggest factor
is darkness.  90% percent of a driver's reaction depends on
vision, and we were just not engineered to see very well in the
dark.

The American Automobile Association and the National Safety Council,
among others, have published critically important suggestions for
improving vision when driving at night, however old you are.
The No. 1 recommendation is to protect your eyes during the day by
wearing sunglasses (neutral-gray lenses are best) and a hat with a
brim when the sun is shining.
Clean the windshield of your vehicle, inside and out, at least
weekly. As with a cloudy lens, a grimy windshield scatters light
and intensifies glare. (You may be surprised by how dirty the
inside of the windshield gets.)
Clean the headlights as well; just a thin layer of grime can reduce
the light they cast by about 90%.  And make sure the headlights
are properly aligned.
Most new cars these days have rear-view mirrors that adjust
automatically at night to eliminate the reflected glare of
headlights behind you. If not, make sure to adjust the mirror
manually to night setting.
If you wear glasses, make sure they are clean. Grimy glasses, like
a grimy windshield, scatter light. When getting a new prescription,
make sure the lenses have an anti-reflective coating.
Avoid looking directly at approaching vehicles at night, even when
their lights are dimmed. Instead, direct your eyes about 20 degrees
to the right, toward the white line on the right side of the road,
and use your peripheral vision to see ahead for those few moments.
Reduce your speed at night and increase the distance between you and
the vehicle ahead of you. You should be able to stop inside the area
illuminated by your headlights. If you overdrive your headlights,
you create a blind crash area in front of your vehicle.

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