It is the darkest time of the day, when it should be hardest to see properly.
But our eyesight is best at dusk, a study has found, because of human beings’ caveman roots.
We see best at 8pm, when twilight falls, and at 8am, in the hours after dawn, according to an experiment.
Before lightbulbs were invented, these were the times when humans were most at risk from nocturnal predators and needed to be able to keep a look-out.
Neuroscientists from Goethe University in Frankfurt found 14 men asked to spot a dim flashing orange cross against a black background, finding they were best able to do so at 8am and 8pm.
Our eyesight is best at dusk, a study has found, due to human beings’ caveman roots (stock)
COULD AN EYE PATCH IMPROVE SIGHT LOSS?
A pioneering eye patch improves vision in people with severe age-related sight loss, research suggested in April 2018.
When implanted at the back of the eye of people suffering from dry macular degeneration, all experienced improved or stabilised vision, a study found.
One woman, aged 69, was even able to read 24 letters on an eye chart after having the device fitted, compared to just seven before, the research adds.
The device involves placing a wafer-thin patch coated with healthy embryonic retinal cells on the tissue near the optic nerve, which sends impulses to the brain where images are formed.
Dry macular degeneration affects around 1.75 million people in the US alone and causes reduced central vision due to thinning of the part of the retina responsible for people’s direct line of sight.
The researchers, from the University of Southern California, placed the patch, measuring 6x4mm, on the retinas of four people with advanced dry macular degeneration.
Each of the participants only had one eye tested, while the other served as a control.
One year on, the patch stabilised the disease in all of the participants’ treated eyes, while those that had not received the patch continued to deteriorate.
Two of the participants were better able to maintain their vision on a single object a year after the implant was fitted.
The researchers believe their findings suggest the patch improves the vision of people with severe age-related dry macular degeneration, at least in the short term.
They plan to conduct a larger trial that tests the device on patients at an earlier stage of the disease.
Cavemen’s brains evolved to spot lions and other predators
Scanning their brains, they discovered the brain improves vision at these times by shutting down background activity in the visual cortex, so it can process the weak signal from a dimly-lit object better.
The study’s co-author, Dr Christian Kell, from the Brain Imaging Centre at Goethe University, said: ‘Driving at dusk and dawn may be easier than it should be because the brain has this mechanism to change our perception of light.
‘This makes sense because it would have helped us to survive as ancient humans by perceiving, for example, a lion in the savannah.
‘It would have kept us safe at critical times when nocturnal predators emerged. The brain also suppresses activity in the hearing region of the brain, which could explain why hearing is better at these times too.’
Eyesight is worse at 2pm
The brains of the 14 males involved in the experiment were scanned at six different times from 8am to 11pm over two days.
Researchers found background ‘noise’ in the visual cortex was reduced at 8am and 8pm. This should make it easier to pick up visual signals by improving the signal-to-background noise ratio.
Indeed, the men did better in the visual test when their brain’s background activity was most reduced.
Asked to press a button when they saw the dimly lit orange cross flashing in front of their eyes, the participants saw the cross on average 32 out of 33 times at 8am and 8pm.
The average at other times was just 30 out of 33 times, with people’s eyesight apparently worst at 2pm. The difference is not in what the men saw through their eyes, but in their visual perception, which is controlled by the brain.
Humans are hardwired to see best at dusk and dawn
Dr Kell said: ‘Whilst the cogs of our inner clock have already been studied in depth, it was not known to date which mechanism optimises visual perception at times when poor signal quality can be expected.’
The researchers did not test the men at dawn, as they did not want to disrupt their sleep, which may also affect brain activity. But they believe their results show humans are hardwired to see best at dusk and dawn, when lighting is at its lowest levels.
The study, published in the journal Nature Communications, says: ‘Humans, a day-active species, largely rely on the visual sense for spatial orientation.
‘Yet, even in ancestral societies, human activity usually extended into morning and evening times of twilight, when natural illuminance is drastically reduced.’
There was up to 26 per cent less activity in the visual cortex of the men in the study at dusk and in the morning than at midday.