Is melatonin useful for more than just sleep?

Some aspects of our behavior (like sleep), cognition, and physical feelings follow a 24-hour rhythmic cycle known as circadian rhythm^[1]. It’s produced by our biological clocks and controls many processes in the body. But it doesn’t exist in a vacuum — it’s influenced by external cues, which are referred to as zeitgebers (German for “time givers”). Light is one of these zeitgebers. It impacts our circadian rhythm through its interaction with blue light receptors in the retina and the neural pathways that they influence.

One of the hormones that light influences is melatonin. Melatonin is one of the primary controllers of our body’s circadian rhythm and is produced in the brain, specifically in the pineal gland in the absence of blue-wavelength light (shown in Figure 1). Therefore, production of melatonin can be disrupted by exposure to artificial light^[2] after sunset. It follows that if melatonin is one of the primary controllers of the circadian rhythm, disruptions to its function may also lead to disruptions in our circadian rhythms^[3].

Several studies in the past few decades have linked circadian rhythm disruptions to negative health outcomes, such as insulin resistance^[4], inflammation^[5], neurological disorders^[6], and cancer^[7]. Many of these disruptions are also linked to exposure to artificial light^[2]. As a result, some organizations^[8] that conduct research on sleep and circadian rhythms recommend avoiding blue light before sleep.

In addition to avoiding bright light, melatonin supplementation has been explored as a way to entrain circadian rhythms. Randomized trials have found that supplementation was able to help manage delayed-sleep phase disorder^[9] and jet lag^[10], conditions believed to be a result of disrupted circadian rhythms^[11]. In addition to melatonin regularizing circadian rhythms, it is a potent antioxidant^[12] and seems to have anti-cancer activity^[13], which has been documented in over a decade’s^[14] worth of research, making it a molecule of interest for oncology researchers.

Although several studies and reviews have been done on melatonin supplementation, few have compiled all the evidence to determine the utility of melatonin for several health outcomes, and few have summarized its mechanisms of action. The study under review is an umbrella review (a review of reviews) that aimed to look at the evidence in favor of the use of melatonin supplements for various health outcomes.

Circadian rhythm abnormalities have been associated with several negative health outcomes. Large cross-sectional and cohort studies have linked artificial light exposure to both negative health outcomes and circadian rhythm abnormalities. Melatonin supplementation has been explored for use in many circadian rhythm disorders and for several other health outcomes. The study under review looked to compile all the evidence on melatonin and assess its impact on various health outcomes.