These days, an inky, sightless night is a rare sight in much of the world. From the harsh glare of streetlamps that make us squint to the gaudy flicker of neon billboards that force sunglass shields, artificial light is blurring the line between day and night.
This has meant birds stay active for almost an hour longer after sundown in brightly lit areas, according to a global study of 583 species and more than 60 million bird vocalisations, drawn from 181 million raw detections.
“I know how I feel when I lose an hour of sleep,” said Brent Pease, the lead author of the study. Dr. Pease is an assistant professor at the School of Forestry and Horticulture at Southern Illinois University Carbondale in the US. “This can’t be a great situation. But the picture is complex.”
Scientists warn this shift disrupts birds’ natural rhythms, altering migration, feeding, and breeding. This in turn unsettles food chains and ecosystems.
For Dr. Pease, this project began as a teaching idea to introduce students to birds via their songs. He set up a simple sound recorder and a small computer to capture bird calls. That’s when he noticed a prompt asking if he wanted to link to a platform where volunteers shared bird sounds identified using artificial intelligence (AI).
“I couldn’t believe what I was seeing,” Dr. Pease said. “There were thousands of sound recorders, all across the globe.”
He likened the discovery of this source to an earlier revolution in wildlife science: trail cameras. Motion- and heat-triggered cameras transformed mammal studies, revealing behaviours once invisible to researchers, from leopards prowling at night to deer grazing at dawn.
BirdWeather, a volunteer-powered acoustic network analysed with an AI model called BirdNET, has done something similar for birds by automatically logging and identifying their songs across continents.
Twilight chorus
“Light pollution in particular has been interesting to me,” said Neil Gilbert, a co-author of the study and a postdoctoral researcher at the Department of Biology, Oklahoma State University in the US. “I’ve thought a lot about how animals are responding to stressors and especially behaviour timing.”
Dr. Pease looped in Gilbert to study how artificial light at night affected bird behaviour. They tapped into this worldwide system of microphones and sensors placed by volunteers in backyards and forests that built a real-time map of bird activity.
The scientists focused on two daily markers: the first song at sunrise and the last at sunset. They also looked at eye size, nest type, migration patterns, and habitats to understand which species were most vulnerable. Then they compared recordings from brightly lit areas with those from darker places.
Dr. Pease and Gilbert thus found that light pollution was stretching the hours for which birds remained active.
By combining millions of recordings from hundreds of species — most from the U.S., Europe, and Australia, with fewer from the Global South — they found that artificial light was nudging birds to stay awake longer by nearly an hour.
“We were pretty surprised by the size of the effect,” Gilbert said. “Fifty minutes as an average across species was quite a bit more than what we were expecting.”
Not all birds were reacting to artificial light to the same degree, however.
Those with large, watchful eyes shifted the most, singing 35 minutes earlier at dawn and 56 minutes later at dusk in bright areas. Small-eyed species barely stirred from their routine. Open-nesters exposed to the sky felt the glow more than cavity nesters sheltered in tree holes. Migratory birds were also more unsettled than stay-at-home species. The changes were sharpest in the breeding season.
Some species were active two hours longer, others by just a few minutes. But the signal was consistent: where nights were brighter, days stretched unnaturally longer.
“If they’re spending an extra hour of time … they’re going to need more additional caloric intake for all this extra activity that they’re doing,” Dr. Pease said. “But this additional extra hour of activity can also result in an increased foraging time and potentially even increased reproductive output.
‘Darkness disrupted’
Anusha Shankar at the Tata Institute of Fundamental Research, Hyderabad, is an integrative biologist who combines insights from across biology to study organisms. She explained why this study matters.
“Every organism that we know of on earth, every animal, even many plants, and phytoplankton — they all have some sort of sense of time,” Dr. Shankar said. “And a lot of that comes from light cues.”
Artificial light breaks these natural cycles. For instance, across Indian cities, birds are dying after colliding with brightly lit glass facades, turning shiny office buildings into deadly traps.
“So if you disrupt these ancient rhythms, everything gets messed up,” she said.
These mismatches can be devastating for migratory birds. Birds time their movements to food sources that also depend on seasonal light cues. If plants flower earlier or insects hatch later, journeys that evolved over time may suddenly become perilous.
Filmmaker Sriram Murali supports keeping the night dark for people, animals, and stars. Through his research and films on fireflies, Murali has campaigned to raise awareness of light pollution. He calls these beetles the “flagship species of the night”. Fireflies use flashes of bioluminescent light to attract mates.
In southern Indian forests and villages, their hypnotic synchronised twinkling can be seen for a brief spell in May and June, just after sunset. On peak nights, entire groves shimmer with their green-gold pulses, like fairy lights strung across the trees. But artificial lighting hacks this communication, making it harder for them to survive.
“We know that exposure to lighting affects the circadian rhythm,” said Mr. Murali of our bodies’ natural clock that controls sleep and wakefulness. “Doctors know it and we are careful about it for our own children. So then the question is, how do we teach people empathy and bring about change?”
A simple switch
Whether stretching a bird’s day is a blessing or a burden is still uncertain. More time to feed and to mate may help; less time to rest may hurt.
The good news is that unlike many other human pressures, light pollution is reversible. Streetlamps can be shielded, billboards dimmed, and lights switched off when not needed.
Dr. Pease pointed out that unlike climate change or habitat destruction, reducing light pollution doesn’t require massive investments or decades of restoration.
“If light pollution turns out to be a negative thing for bird populations, we can do a behaviour change across the globe to turn out the lights and make the night dark again,” Dr. Pease said. “We’re just a light switch away here.”