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Examining the effects of melatonin on cognition

Background

Melatonin is a naturally occurring hormone secreted by the pineal gland in the brain. Although melatonin can promote better sleep, its effects on cognitive function aren’t clear. Endogenous levels of melatonin are inversely associated with age, which may explain why insomnia appears to be more common later in life. Insomnia is also associated with increased risk for Alzheimer’s disease (AD), in which lack of sleep is thought to increase severity of the disease by promoting increased formation and decreased clearance of protein aggregates in the brain. This study examined cognitive outcomes of studies testing the effect of melatonin supplementation in participants with AD and insomnia and in healthy individuals.

The study

This meta-analysis evaluated 22 randomized controlled trials (9 on AD, 2 on insomnia, and 11 in healthy participants) with a total of 746 participants. The trials on AD included 569 total participants with a mean age of 79 years. The effects of melatonin on cognition were examined in a total of 50 participants with insomnia with a mean age of 65. The studies in healthy participants involved 182 total participants with a mean age of 29. The melatonin doses tested ranged from 5 mg to 80 mg, and some of the trials administered melatonin during the day or afternoon to test the effects on memory.

The results

AD patients taking melatonin for more than 12 weeks showed significant improvements in the Mini-Mental State Examination (MMSE) score, but shorter-term studies failed to show improvements in global cognitive function. Melatonin specifically improved cognitive function in patients during the mild stages of AD. In healthy participants, daytime melatonin treatment decreased their accuracy score but didn’t affect reaction time or memory. The studies on insomnia were not subjected to meta-analysis due to differences in study design, but one study indicated that long-term melatonin supplementation at night improved memory, and another indicated that melatonin improved daytime cognitive function compared to a placebo.

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