Melatonin is a peptide hormone secreted in the brain (mostly) and regulates sleep (for the most part). Sunlight acts on the retina to actively suppress its synthesis during the day, and the gradual loss of sunlight and this loss of suppression causes a spike near night which facilitates sleep. But melatonin appears to be a lot more than just a sleep regulator and in looking at correlates of (1) overall melatonin secretion levels (area under curve) throughout a day and (2) a proper circadian rhythm of high nightly melatonin and low daily melatonin, it appears that a properly controlled circadian rhythm is correlated with many beneficial health states and longevity itself.
(An interesting study for this latter claim can be found here: http://www.ncbi.nlm.nih.gov/pubmed/17764865 where average youth had drastic differences between daily and nightly melatonin and this was preserved in centurions; average but healthy adults seemed to have a normalization of this cycle, and a normalization also appears to occur in smokers based on limited evidence)
The most interesting part of melatonin, as it applies to my interests (putting stuff into my face to manipulate my own health) is the ability of very low doses (500mcg, or half of a milligram) and economical doses of melatonin to actually entrain the circadian rhythm. Afraid that your lifestyle will disrupt this ‘ideal’ rhythm? Just override it with supplemental melatonin!
This ‘override’ of internal regulation appears to be reliable and powerful. It is most well known for ‘curing’ jet lag, which is a disruption between what the body says (internal regulation of time) and what the sun is currently saying (external regulation of sleep) and is also used in shift work as well as helping entrain the circadian rhythm of blind persons (who lack external regulation, since they cannot perceive light). It is quite a powerful hormone in this regard.
Other interesting benefits of melatonin supplementation include:
– It does not appear to be subject to negative feedback (a phenomena of supplementing hormones that your body makes, where natural production of the hormone decreases because you don’t ‘need’ any more than you are ingesting; a reason testicles shrink nearing the end of a cycle of testosterone injections)
– It has quite a short half-life and time in the body, acting in 30 minutes or so and pretty much leaving the body within 2-4 hours. Even very high doses (80mg) appear to be fully excreted in less than a day, so there is no build-up apparent
– On those line, toxicity is very high. 500mcg (0.5mg) is an active dose, yet many studies that want to see melatonin’s affects on hormones used up to 160-200 times this dose with no apparent side-effects aside from sedation
– Finally, this stuff knocks you right out for a good night sleep and forms the basis of any ‘relaxing’ supplement stack
Like usual, more information can be read on Examine.com at http://examine.com/supplements/Melatonin. In sum though, it appears Melatonin is more than just a sedative to use at night but that the ability of melatonin to entrain the circadian rhythm may unlock a variety of health benefits (I am hesitant to say prolong life, but it could prevent a lifestyle-induced shortened life).