Streetlights may make it easier for humans to travel by road, but they could cause a problem for "commuting" bats, say researchers.
Scientists have found that, as bats travel to feeding grounds, they avoid hedgerows illuminated by streetlights.
Reporting in the journal Current Biology, they say this could cause bats to use longer and less safe routes.
The researchers studied the effect with artificial lights along flight routes used by lesser horseshoe bats.
Emma Stone, a biologist from Bristol University, UK, who led the study, placed the experimental lights along hedgerow-lined flight-paths used by the bats when they leave their colonies.
These lights mimicked the colour and intensity of ubiquitous sodium streetlights, which are used throughout the world.
"The magnitude of the effect was surprising," said Professor Gareth Jones, one of the authors of the study.
"With the lights on, there was about a quarter to an eighth of the activity - or number of bats flying along the route - compared to when the lights were off."
Professor Jones explained that, although the bats have sensitive hearing, which they rely on for navigation, it is not tailored to help them avoid predators.
"Echolocation is of limited value for detecting predators, because the high frequencies they use are directional, and limited in range," he said.
This means the bats are vulnerable to attack from birds of prey if they fly in lit conditions.
Avoiding predators, Professor Jones said, was probably the main reason why bats were nocturnal. And relatively slow-flying lesser horseshoe bats, in particular, seem to be "hard-wired" to avoid light.
The researchers suggest this finding could be considered in conservation measures; light could be deviated away from commuting routes with trees and sheltered areas near colonies.