Melatonin: the body's natural sleep-promoting hormone

Prescriptions for short periods are considered safe, but there is little research into the long-term effects

More than 6mg of melatonin a night showed little extra benefit, a recent study found. Photograph: Sarah Lee/The Guardian

In daylight hours there is so little melatonin in the bloodstream that it is barely detectable. But when the sun goes down, the eyes sense the failing light, and part of the hippocampus signals the pineal gland, a pea-sized lump of tissue near the centre of the brain, to ramp up production of the sleep-promoting hormone. Levels of melatonin rise sharply from 9pm, inducing feelings of sleepiness, and remain high until the following morning.

Much of the research on prescribing melatonin for children with sleep problems has focused on those with disorders such as autism, ADHD and intellectual disability (ID). For good reason too: sleeping difficulties are far more common and pronounced in children with neurodevelopmental or psychiatric disorders. For them, small doses of melatonin can be safe and effective. In one recent study, researchers from Southampton University monitored the sleep patterns of 45 children with autism, ADHD, or ID, and found that a third fell asleep faster, slept longer, and woke less frequently at night on low dose (2.5-3mg) melatonin. Above 6mg per night there was little extra benefit.

A poor night’s sleep can be caused by any number of factors, but there is good evidence that screen time matters, whether it is TV, computer, tablet or mobile phone. A recent review of scientific papers on the issue found that 90% linked screen time to poor sleep in schoolchildren and adolescents. Part of the problem is obvious: being online at bedtime eats into the hours left for sleep, and it hardly helps people to wind down for the night. But glowing screens can affect sleep directly by suppressing the natural production of melatonin. Using an iPad on full brightness for two hours, for example, has been shown to suppress melatonin levels.

Taking melatonin over the short to medium term has a number of known side effects, but they are not considered serious. Some people develop headaches, dizziness and stomach cramps, or feel sleepy in the day, irritable, or have transient feelings of depression. It should be avoided, however, in children with immune disorders or those having therapies that alter the immune system. What is unclear is the long-term safety of taking the hormone, because the relevant studies have not been done.

The NHS advises parents to ensure their children’s bedrooms are screen-free zones that are dark, quiet and tidy, well-ventilated, and between 18 to 24C. It suggests a healthy routine to promote good sleep: a warm, but not hot, bath before bed, dimmed lights to encourage children to produce melatonin, and once they are in bed, the chance to read quietly, or have a story read to them.