The number of Americans taking the supplement has doubled in five years—but it is controversial and not available over the counter in the UK, reports The Guardian.

Melatonin use is less common in the UK, where it is only licensed in slow-release form to treat sleep disorders in those aged 55 or over, and can’t be bought over the counter. Supplements are sometimes prescribed off-label to children with sleep problems associated with conditions including attention deficit hyperactivity disorder (ADHD) and autism.