The increasing popularity of light-emitting diode lights might have a downside for human health, researchers say.

It has been know that "white" artificial light reduces melatonin production in the pineal gland.

However, researchers have now qualified for the first time the degree of suppression depending on a person's exposure to different light sources.

The multi-country study, published in the Journal of Environmental Management, compared high pressure sodium (HPS) bulbs, where give off an orange-yellow light and have been used in street lighting, with 'white' light bulbs that actually give off blue light.

Researchers found that white-light metal halide bulbs, which are often used in stadium lighting, suppress melatonin ad a rate more than three times greater than the HPS bulb, while white LED bulbs suppress melatonin at five times the rate of the HPS bulb.
Incandescent bulbs suppress melatonin at only 2.5 times the rate of HPS bulbs.

Melatonin was important for adjusting the biological clock, and had a documents antioxidant and anti-cancer properties, the researchers said.

The current move to white lamps will likely increase melatonin suppression in humans and animals, making night-time lighting a public health issue warranting regulation like 'classic' pollutants, they warned.