Sunshine acts like an addictive drug and has a similar effect on the body as heroin, scientists claim.
Ultraviolet (UV) rays from the sun stimulate the production of endorphins, "feel good" hormones that act on the same biological pathway as opioid drugs, researchers at the USA's Harvard Medical School argue.
The study exposed laboratory mice to sunlight and then tested their hormone levels and reactions.
Having been exposed to UV light for six weeks, the mice exhibited withdrawal symptoms when it was taken away.
Researchers say the mice's reactions suggest that people's desire to bake for hours on a beach may appease our craving for a sunshine "fix", in much the same way as an addict satisfies a yearning for heroin or morphine.
However, some British scientists say mice are not sufficiently similar to humans for the results to be valid, and have urged caution.