Grape skins used to make red wine are rich in melatonin, Italian scientists have found. The findings could help explain why people reach for a bottle of red wine to wind down at night, since the hormone tells the body when it's time to sleep.

Grape skins rich in the compound included merlot, cabernet sauvignon and sangiovese, the main ingredient in Chianti, the researchers report in Monday's online issue of the Journal of the Science of Food and Agriculture.

Scientists have found the skin of grapes used to make red wine contains melatonin, a hormone that affects sleep cycles.
(CBC)

The skins are removed in making white wine.

Until recently, it was thought only animals made melatonin. The hormone is naturally secreted by the brain's pineal gland. It affects internal clock and sleep cycles.

"The melatonin content in wine could help regulate the circadian rhythm [sleep-wake patterns], just like the melatonin produced by the pineal gland in mammals," study author Iriti Marcello said in a release.

Melatonin is also an antioxidant that mops up free radicals, preventing damage to cells and tissues.

But animals can't synthesize an essential amino acid, tryptophan, needed to make melatonin. Animals get tryptophan from their diet. 

Marcello and his colleagues studied the melatonin content in the skin of eight varieties of grapes.

The levels varied greatly. Nebbiolo grapes, used to make Barolo and Barbaresco wines, were highest.

Given that the amounts in all grape varieties tested except cabernet franc contained more melatonin than usually found in our blood, the levels are "significant" for testing as a natural health product,  the team concluded.

Boosting levels

In plants, melatonin also regulates seasonal and circadian rhythms and acts as an antioxidant. Treating grape vines with a vaccine called benzothiadiazole enhanced synthesis of melatonin while helping to protect the plants from a fungus, the researchers said.

They are conducting more experiments to verify if melatonin can also be detected after the grapes are fermented for wine and noted the hormone seems to be absorbed better when dissolved in alcohol.

But Richard Wurtman of the brain and cognitive science department at Massachusetts Institute of Technology remained unconvinced that the researchers were in fact measuring melatonin.

More sophisticated chemical tests are needed, he told Chemistry & Industry magazine.

There is also a question of whether alcohol reduces the quality of sleep after making people feel drowsy, said Dr. Brian Murray, a neurologist who specializes in sleep medicine at Sunnybrook Health Sciences Centre in Toronto.

The melatonin findings are interesting, but the hormone isn't a major player in inducing sleep, Murray told CBC Newsworld.