Melatonin (N-acetyl-5-methoxy tryptamine) carries out a vast array of tasks; it also has the potential to be medically useful in a number of conditions, including radiation exposure, Alzheimer's disease and tinnitus.
Melatonin has been lovingly conserved by evolution and can be found in animals, plants, fungi and bacteria. In animals, it is described as a hormone and plays a number of important roles, the most well-known of which is the maintenance of circadian (daily) rhythms.
In this article, we will look at melatonin's role in the human body and how it is used medically.
Contents of this article:
Fast facts on melatonin
Here are some key points about melatonin. More detail and supporting information is in the main article.
- Melatonin was first discovered in 1958
- Initially, melatonin was extracted from cow pineal glands
- Melatonin is most famous for regulating the body's internal clock
- Melatonin is involved in many processes, including the regulation of certain immune responses
- Melatonin supplements cause very few side effects but they can interact with other drugs
- Some evidence suggests melatonin might relieve certain symptoms of Alzheimer's disease
- Melatonin has strong antioxidant capabilities
- The primary medical use of melatonin is to treat sleep disorders
- Melatonin might help ward off damage caused by radioactivity.
What is melatonin?
Melatonin's chemical structure allows it to travel through the systems of the body with ease.
Although melatonin is most commonly called a hormone, it could just as easily be referred to as an autacoid (a biological factor that acts like a local hormone), a chronobiotic (an agent that causes changes to the body clock), a hypnotic (a sleep inducer), an immunomodulator or a biological modifier. Such is the scope and variety of this all-pervasive substance.
Melatonin is amphiphilic, meaning that it has hydrophilic (water-loving) and lipophilic (fat-loving) properties. These properties enable melatonin to pass easily into any cell, fluid or compartment within the body.
Melatonin was initially discovered in 1958 by Lerner, Case and Takahashi. They extracted it from the pineal glands of cows and gave it the name we use today. They identified melatonin's skin-lightening capabilities in frogs and fish and hoped that it might be useful in the treatment of skin conditions.1
In the 1970s, Lynch and colleagues found that melatonin was released from the pineal gland in a circadian rhythm.2 Over the years, the secrets of melatonin's role in regulating the sleep/wake cycle have slowly been revealed. Later, in the 1990s, melatonin's antioxidant properties were discovered.3
The role of melatonin in circadian rhythms
Circadian rhythms are generated within the hypothalamus.
Of melatonin's many roles, the most well understood is the part it plays in maintaining circadian rhythms. From the third month after birth, levels of melatonin enter a cyclical pattern that will continue for the rest of an individual's life.
In mammals, the circadian "clock" resides in two groups of cells within the hypothalamus, referred to as the suprachiasmatic nucleus (SCN). Using the daily cycle of light and dark, the SCN creates and maintains a daily cycle.
Certain hormones are released preferentially at specific times of the day. In the late afternoon and early evening, the body gets ready for sleep. During the early hours of the morning, the body starts to prepare for waking and activity.
Information regarding light levels reaches the SCN from a set of special light receptors in the retina (separate to the rods and cones most people are familiar with). This light information is relayed to the brain even when the eyelids are closed. The timing of light and dark phases is passed from the SCN on to the pineal gland.
The pineal gland, deep in the center of the brain, responds and releases melatonin at night. Conversely, the release of melatonin is suppressed during daylight.
Even when a human is kept away from all external light sources and time references, the body maintains a near perfect rhythm, typically relaxing into a natural rhythm of 24 hours and 11 minutes. Previous estimates put the natural circadian rhythm at 25 or even 28 hours, but the most recent and accurate results suggest it is astonishingly close to the length of a natural solar day.4
On the next page, we look at some of melatonin's other roles within the human body.