Adjusting to the dark

It's like wearing sunglasses at night

Eyes adapt to dim light or darkness by widening the pupils to let in as much light as possible. The iris, or the coloured part of the eye surrounding the pupil, contains tiny muscles that control the size of the pupil. And as we age, these muscles weaken and do not respond as well to the need to let in more light.

This results in a pupil too small for seeing well in dim light -- sort of like you've put on sunglasses at night.

The less responsive muscles in the iris also affect the eye's ability to adjust to sudden light changes, such as when a car with its headlights on approaches and then passes. A driver may also experience a decrease in depth perception.

Other age related changes include the formation of cataracts, which makes the lens less transparent and reduces the amount of light reaching the retina. Cataracts can cause light to scatter, resulting in the temporarily blinding glare from the headlights of an approaching vehicle.

To improve night vision, experts advise protecting your eyes during the day by wearing sunglasses and a hat with a brim when the weather is sunny. Bright sunlight can bleach the photoreceptors and thereby, lengthen the time it takes for your eyes to adjust to the dark. Normally it takes 30 to 60 minutes for full adaptation to the dark, but being in bright sunlight for two or three hours can delay this adaptation by hours.

Road safety

Traffic deaths are three times more frequent at night than during the day, even though only 20 per cent of driving is done after dark. While fatigue and alcohol can contribute to this risk, experts say the biggest factor is darkness. A full 90 per cent of a driver's reaction depends on vision.