Lying around in the sun all day is about far more than just the tan. Sunshine acts like an addictive drug and baking on the beach for hours helps appease our craving for a sunshine "fix".

The sun's ultraviolet (UV) rays have a similar effect on our bodies as heroin, scientists claim, and by topping up our tan we're acting in much the same way as an addict satisfying a yearning for heroin or morphine.

Experts knew that sun-seeking behaviour could fit the clinical criteria for a substance-related disorder, but what underlay this apparent "addiction" had been unknown until now.

Lead scientist Dr David Fisher, from Harvard Medical School in the US, and his team investigated links between UV exposure and the opioid receptor pathway in "naked" laboratory mice.

After a week in the artificial sunshine, endorphin levels in the blood of shaved animals increased.

At the end of six weeks, the mice were given an opioid-blocking drug, naloxone. Abruptly denied the drug-like effects of UV, they suffered an array of withdrawal symptoms, including shaking, tremors and teeth chattering.

In addition, UV exposure caused the animals' tails to stiffen and lift up - an effect also seen when mice are given opioid drugs. When the mice were removed from the UV rays the symptom, known as "Straub tail", gradually faded away.

"It's surprising that we're genetically programmed to become addicted to something as dangerous as UV radiation, which is probably the most common carcinogen in the world," said Dr Fisher, whose findings appear in the journal Cell.

But British experts urged caution when extrapolating the results of the research to humans.

Dr Clare Stanford, reader in experimental psychopharmacology at University College London, said the study's evidence fell short of showing addiction to UV light in mice "and it is even less certain that the work predicts addiction in humans".

While Dr Richard Weller, senior lecturer in dermatology at the University of Edinburgh, added: "Mice are nocturnal animals, covered in fur, which avoid the light, so one must be cautious about extrapolating from these experiments to man."