Before sunrise, birds begin to sing. But now, they’re starting earlier and finishing later—thanks to us.

A new study published in Science finds that artificial light is shifting birds’ daily routines around the world. Researchers analyzed over 60 million recordings from 583 bird species and found that birds in brightly lit areas sing for nearly an hour longer each day—starting 18 minutes earlier and ending 32 minutes later than those in darker places.

“We were shocked by our findings,” Brent Pease, assistant professor at Southern Illinois University, told The Guardian. “Under the brightest night skies, a bird’s day is extended by nearly an hour.”

A Global Chorus, Shaped by Human Light

The research is rooted in one of the most ambitious behavioral datasets ever assembled. Working with BirdWeather, a global citizen science platform, Pease and co-author Neil Gilbert, an assistant professor of biology at Oklahoma State University, used machine learning to identify vocalizations submitted from thousands of locations worldwide.

They ultimately focused on 4.4 million recordings of morning songs and 1.8 million evening calls. To gauge light exposure, they overlaid these data points with satellite images from NASA’s VIIRS (Visible Infrared Imaging Radiometer Suite) instruments, which measure nighttime brightness from space.

“BirdWeather unlocked behavioral research at large geographic and timescales,” Pease explained. “We could start to learn—at a scale never [done] before—how birds were responding behaviorally to human forces.”

What they found is that light pollution nudges birds into a slightly earlier song and stretches their active hours across space, species, and seasons.

Some Birds Sing More Than Others

Not all species respond to artificial light in the same way.

Birds with larger eyes, which may be more sensitive to low-light cues, were especially impacted. Species like the American robin, Northern mockingbird, and European goldfinch showed longer vocalization periods than average. Meanwhile, birds with smaller eyes, such as sparrows, were less responsive.

The effect was also amplified during breeding season, when birds naturally wake and sing earlier to compete for mates. In cities and other well-lit areas, artificial light may trick them into thinking dawn has arrived, pushing them to begin even earlier.

“It might disrupt their sleep, though they might compensate by sleeping during the day,” said Gilbert, as per ScienceNews. “Extra activity may even be helpful, giving birds more foraging time to feed young.”

What Does This Mean for Birds?

That’s the complicated part.

According to Pease and Gilbert, this prolonged activity may affect birds’ survival and reproduction rate in more than one way. Fifty minutes of additional activity a day may lead to reduced resting time, potentially resulting in higher caloric needs.

Sleep deprivation, could impair immune function or breeding success, much as it does in humans. On the other hand, more daylight-like hours could mean more time to find food or mates, possibly improving fledgling survival in some species.

“Birds are different,” said Pease. “They have developed interesting strategies to cope with loss of sleep during migratory periods.”

But the concern is growing. Light pollution now affects 23% of Earth’s surface, and it’s rising in both extent and intensity. More than 80% of the world’s population, and 99% of U.S. and European residents, now live under light-polluted skies.

“If the extended day is resulting in sleep debt, as it often does for humans,” Pease and Gilbert wrote, as per Gizmodo “then we might expect adverse health or population outcomes, further exacerbating the long-term decline of bird populations across the globe.”

Scientists have long known that light pollution disorients migrating birds, leading to fatal collisions with buildings. This study—unusual for its scale and detail—shows that even the glow of streetlamps and windows is reshaping birds’ daily routines.

“Our lights, which we cast more or less mindlessly into the night,” said Pease, “are having widespread and often subtle effects on the lives of animals all around us.”