In the UK, melatonin is a prescription-only medicine. In the US, however, it is widely available over the counter as a dietary supplement, often in doses ranging from a few milligrams up to 20mg which is 10 times the dose recommended for jet lag in the UK.

So what happens if you take it every night?

The evidence available so far is relatively reassuring. Studies have not found clear evidence that melatonin loses its effectiveness with continued use, and a review examining studies of at least six months' duration found a low frequency of adverse events associated with longer-term use.

Reported side effects are generally mild and can include daytime sleepiness, headache, dizziness and nausea.

A recent observational study raised concerns about a possible association between long-term melatonin use and heart failure, finding a higher incidence of heart failure among people with insomnia who had used melatonin for at least a year. However, the study cannot establish that melatonin caused this increased risk, and the findings have not yet been confirmed in high-quality clinical trials. A subsequent BMJ article examining the issue found that melatonin users differed substantially from non-users, suggesting that other factors may partly explain the observed association with heart failure.

However, there is an important caveat: there are relatively few high-quality, double-blind trials investigating people taking melatonin continuously over long periods. This means that the absence of evidence of serious harm shouldn't be interpreted as definitive proof that taking melatonin every night for years is completely risk-free.