Not just the Sun, but the light travelling from galaxies across the universe for billions of years is also responsible for our suntans, scientists say.
We are constantly bombarded by about 10 billion photons per second from intergalactic space when we are outside, day and night, a new study has found.
When we lie on the beach, our bodies are bombarded by about sextillion photons of light per second, researchers said.
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Most of these photons, or small packets of energy, originate from the Sun but a very small fraction have travelled across the universe for billions of years before ending their existence when they collide with your skin, they said.
Astronomers accurately measured the light hitting Earth from outside our galaxy over a very broad wavelength range.
The research looked at photons whose wavelengths vary from a fraction of a micron (damaging) to millimetres (harmless).
However, radiation from outside the galaxy constitutes only ten trillionths of your suntan, so there is no immediate need for alarm, researchers said.
"Most of the photons of light hitting us originate from the Sun, whether directly, scattered by the sky, or reflected off dust in the Solar System," said Simon Driver, Professor at the International Centre for Radio Astronomy Research (ICRAR), who led the study.
"However, we're also bathed in radiation from beyond our galaxy, called the extra-galactic background light," Driver said.
"These photons are minted in the cores of stars in distant galaxies, and from matter as it spirals into supermassive black holes," he added.
Driver, who is based at the University of Western Australia, measured this ambient radiation from the Universe, from a wide range of wavelengths by combining deep images from a flotilla of space telescopes.
While 10 billion photons a second might sound like a lot, Professor Driver said we would have to bask in it for trillions of years before it caused any long-lasting damage.
Rogier Windhorst, from Arizona State University, said the Universe also comes with its own inbuilt protection as about half the energy coming from the ultraviolet light of galaxies is converted into a less damaging wavelength by dust grains.
"The galaxies themselves provide us with a natural suntan lotion with an SPF of about two," he said.
The study was published in the Astrophysical Journal.
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