Scientists in the US say the sun could be as addictive as heroin. They have been studying why people crave good weather.

Rays of sunlight apparently trigger feel-good endorphins in the body which people can get hooked on.

The ultraviolet rays stimulate the hormones that act on the same biological pathway as opiates.

The study from Harvard Medical School suggests that the desire to bake for hours on a beach involves more than topping up a tan.

The scientists found that UV-habituated mice show withdrawal symptoms if endorphin activity is blocked following exposure.

Several studies - particularly those enrolling people who use indoor tanning facilities like sunbeds - have found evidence of addiction-like behaviour in frequent tanners.

For example, frequent tanners were somehow able to tell the difference between tanning beds using UV radiation and those delivering non-UV light.

Other studies have found that giving an 'opioid blocker' produced withdrawal-like symptoms in frequent tanners.

The investigators delivered a daily dose of UV light - equivalent to the exposure of fair-skinned humans to 20 to 30 minutes - on a group of mice for six weeks. The dose was calculated to induce tanning but not burning of their skin.

Within a week of the first UV exposure, levels of beta-endorphin in the animals' blood rose significantly, staying elevated during the study and gradually returning to normal after UV exposure was stopped.

Dr. David Fisher is behind the research.

Dr. Paul Moss is head of the department of cancer services at the University of Birmingham. He says sunlight is good for you, but in moderation.