Melatonin, a little more important than you think.
In humans,
melatonin is produced by the pineal gland, a gland about the size of a pea, located in the center of the brain. The melatonin signal forms part of the system that regulates the circadian cycle by chemically causing drowsiness and lowering the body temperature, but it is the central nervous system (more specifically, the suprachiasmatic nucleus) that controls the daily cycle in most components of the paracrine and endocrine systems rather than the melatonin signal (as was once postulated).
Production of
melatonin by the pineal gland is inhibited by light and permitted by darkness. For this reason melatonin has been called "the hormone of darkness" and its onset each evening is called the Dim-Light Melatonin Onset (DLMO). Secretion of melatonin as well as its level in the blood, peaks in the middle of the night, and gradually falls during the second half of the night, with normal variations in timing according to an individual's chronotype.
Until recent history, humans in temperate climates were exposed to only about six hours of daylight in the winter. In the modern world, artificial lighting reduces darkness exposure to typically eight or fewer hours per day all year round. Even low light levels inhibit melatonin production to some extent, but over-illumination can create significant reduction in
melatonin production. Since it is principally blue light that suppresses melatonin, wearing glasses that block blue light in the hours before bedtime may avoid melatonin loss. Use of blue-blocking goggles the last hours before bedtime has also been advised for people who need to adjust to an earlier bedtime, as melatonin promotes sleepiness. Melatonin levels at night are reduced to 50% by exposure to a low-level incandescent bulb for only 39 minutes, and it has been shown that women with the brightest bathrooms have an increased risk for breast cancer. Reduced melatonin production has been proposed as a likely factor in the significantly higher cancer rates in night workers, and the effect of modern lighting practice, including light pollution, on endogenous melatonin has been proposed as a contributory factor to the larger overall incidence of some cancers in the developed world.
AntioxidantBesides its primary function as synchronizer of the biological clock, melatonin may exert a powerful antioxidant activity. In many lower life forms, it serves only this purpose. Melatonin is an
antioxidant that easily can cross cell membranes and the blood-brain barrier. Melatonin is a direct scavenger of OH, O2?, and NO. Unlike other antioxidants, melatonin does not undergo redox cycling, the ability of a molecule to undergo reduction and oxidation repeatedly. Redox cycling may allow other antioxidants (such as vitamin C) to regain their antioxidant properties. Melatonin, on the other hand, once oxidized, cannot be reduced to its former state because it forms several stable end-products upon reacting with free radicals. Therefore, it has been referred to as a terminal (or suicidal) antioxidant. Recent research indicates that the first metabolite of melatonin in the melatonin antioxidant pathway may be N(1)-acetyl-N(2)-formyl-5-methoxykynuramine or AFMK rather than the common, excreted 6-hydroxymelatonin sulfate. AFMK alone is detectable in unicellular organisms and metazoans. A single AFMK molecule can neutralize up to 10 ROS/RNS since many of the products of the reaction/derivatives (including melatonin) are themselves antioxidants. This capacity to absorb free radicals extends at least to the quaternary metabolites of melatonin, a process referred to as "the free radical scavenging cascade". This is not true of other, conventional antioxidants. In animal models,
melatonin has been demonstrated to prevent the damage to DNA by some carcinogens, stopping the mechanism by which they cause cancer. It also has been found to be effective in protecting against brain injury caused by ROS release in experimental hypoxic brain damage in newborn rats. Melatonin's antioxidant activity may reduce damage caused by some types of Parkinson's disease, may play a role in preventing cardiac arrhythmia and may increase longevity; it has been shown to increase the average life span of mice by 20% in some studies.
