Drop into any pharmacy in America and you will find jars of melatonin promising to relieve you of dreaded jet lag. There are tablets, pink gummies, potent-looking capsules—whatever appeals. But all you want to know is: does it work?

Melatonin is known as the “darkness hormone”. When the sun goes down, it is released by the pineal gland in the brain. Production peaks in the middle of the night before slowly falling as the morning light returns. Although driven by the biological clock, rather than darkness itself, the emergence and disappearance of light helps regulate the clock each day and keeps melatonin production synchronised to the day-night cycle. The night-time increase in melatonin puts people into a pleasant state that makes it easier for them to fall asleep. And when people are given melatonin during the day, they get sleepy then, too.

Jet lag, similarly, can cause melatonin disruptions. One reason is that sleep is interrupted by the bright lights of the plane cabin, but the much more detrimental effect comes from arriving at a destination with a day-night cycle out of sync with your biological clock. The clock, and the melatonin, can take days to catch up.

Managing jet lag with melatonin supplements, therefore, has become popular. But understanding whether or not they work is hard. Experiments that mess with people’s biological clocks in a controlled way, and which also recreate real-life scenarios, are not easy to do. Many studies, for instance, have kept people from sleep by exposing them to bright lights all night. But that is not only a little cruel, it is not an ideal way to model sleep disruption.

Instead, scientists have given melatonin to people who were travelling anyway, such as air cabin staff, soldiers and scientists travelling to conferences. In those cases, the supplements do seem to work. A landmark paper that pooled the results of five randomised controlled trials in 2002 found that people given melatonin rate their jet-lag experience as half as bad as those who are given a placebo, on a scale from zero to 100.

“Melatonin is quite effective if you have to speed up your clock,” says Derk-Jan Dijk, director of the Surrey Sleep Research Centre. “It’s not good at slowing down your clock.” That means taking melatonin may be more useful after eastward flights, when you have to go to bed sooner than your body wants to, than after westward flights, when you have to stay up. Fortunately for those with westward travel plans, there are other things you can do to help your biological clock adjust—gradually shifting the wake-sleep cycle in the days before the flight, for example, and getting natural sunlight and exercise during daytime hours at the destination, which helps adjust melatonin production.