Melatonin is a lipophilic hormone synthesized from serotonin by the pineal gland and has been shown to be involved in hormone release and is most well-known for regulating sleep . One of the most striking characteristics of this hormone is that it is secreted only during the night, or more exactly, in darkness. Therefore, melatonin concentrations in plasma are low during the day (light) and reaches peak value of about 1 nM during the night (darkness) . It is often referred to as the hormone of darkness.
Melatonin is indoleamine (family of monoamine neurotransmitters) and is derived from essential amino acid tryptophan . It is synthesized by the pineal gland during the night while during the day production basically halts. Although the pineal gland is the primary source of melatonin release into the systemic circulation, there are a number of extrapineal sources of melatonin synthesis including the retina, gastrointestinal tract, bone marrow, circulating lymphocytes, blood platelets and skin . Light–dark sensations at the retina are passed to the hypothalamus. Fibres from the hypothalamus descend to the spinal cord and ultimately project to the superior cervical ganglia, from which sympathetic postganglionic neurons ascend back to the pineal gland . Once in circulation it has a short half-life and it is primarily metabolized in the liver where it is converted by cytochrome P450 enzymes to its main metabolite, 6-sulfatoxymelatonina that is excreted in the urine. The amount produced during the night appears to be greatest around the time just before puberty with a steady decrease thereafter through middle and old age .
Meltaonin is not only produced in the pineal gland but is also naturally present in edible plants . Very small amounts of it are found in foods such as meat (lamb, beef, pork, chicken, and fish), grains, fruits (cherries, strawberries, raspberries, goji berries, tomato), and vegetables.
In adults, melatonin is taken in doses from 0.2 mg to 20.0 mg, based on the reason for its use. As a sleeping aid, doses between 0.5 mg and 5 mg seem appropriate. However, in studies significant hypnotic effects are seen with doses as low as 0.1 mg. For boosting growth hormone 5 mg seems more effective than smaller doses.
Melatonin’s Interaction with Growth Hormone
It has been shown that oral melatonin ingestion at doses of 0.5 mg  and 12 mg  increases growth hormone in males. It has also been show that melatonin and resistance training alone increase the levels of growth hormone .
Erika Nassar et al.  decided to determine the effects of ingestion of a single dose of melatonin and heavy resistance exercise on serum growth hormone, somatostatin, and other hormones of the growth hormone/insulin-like growth factor 1 (IGF-1) axis. The researches tested sixty apparently healthy, resistance-trained (regular resistance training at least twice weekly for at least one year) males [n = 30] and females [n = 30] in the study. None of them consumed any nutritional supplements (excluding multi-vitamins) 2 months prior to the study. They were randomly assigned to ingest either a melatonin supplement at 0.5 mg or 5.0 mg, or 1.0 mg of dextrose placebo. As participants ingested the supplement they underwent blood sampling every 15 min for 60 min, at which point they underwent a single bout of leg press for 7 sets of 7 reps at 85% 1-RM. In males 5.0 mg of melatonin caused growth hormone to increase (p = 0.017) and somatostatin to decrease prior to training (p = 0.031), whereas both 0.5 and 5.0 mg melatonin were greater than placebo after exercise (p = 0.045)(Fig. 1) and less than placebo for somatostatin. No significant differences occurred for insulin-like growth factor 1. This study was supported by Health Sciences, Inc. (multilevel marketing company that produces various nutritional and skin-care products).
Fig. 1 – Free growth hormone for males (A) and females (B) expressed as the delta values for the peak changes pre- and post-exercise relative to baseline values. Significantly different from placebo (p < 0.05). † Significantly different from the corresponding values for females (p < 0.05).
Meeking et al.  studied the effects of single (5 mg) dose of melatonin on exercise induced growth hormone release in men. An 8 min bout of bicycle exercise was performed at workload corresponding to 70% VO2max. Researchers found that this short exercise induced and increase in growth hormone concentrations following the melatonin administration that was greater from that with the placebo. It was suggested that melatonin has an effect on central hypothalamic regulation through either growth hormone releasing hormone or somatostatin release. Valcavi and colleagues  believe that increased growth hormone response was facilitated at the hypothalamic level through inhibition of endogenous somatostatin release since the melatonin effect was abolished following the administration of pyridostigmine (a cholinergic agonist drug which is likely to suppress hypothalamic somatostatin release).
Another study where 10 health males underwent full-body resistance training with 6 mg melatonin an hour before heavy resistance exercise actually noted decreased growth hormone concentrations induced by exercise .
Effects on Exercise Performance
5 mg of melatonin taken 3 hours before exercise was found to negligibly improve physical performance (4 km cycling time trial and grip strength), reduce alertness, short-term memory and exercise heart rate, whereas perceived exertion was unaffected . Researchers concluded that mental rather than physical components of short-term athletic performance were negatively affected. Atkinsons et al.,  also tested 2.5 mg of melatonin as an ergogenic aid in hot environment. While melatonin ingestion moderated the increase of body temperature, and it amplified the skin blood flow and hypotension responses to exercise, it was not confirmed as an ergogenic aid for endurance performance in hot environments. In this study subjective feelings of sleepiness and alertness were not found to be significantly affected.
Melatonin as Sleep Aid
People use melatonin to adjust the body’s internal clock, for the inability to fall asleep (insomnia), delayed sleep phase syndrome and rapid eye movement sleep behavior disorder.
There is some strong evidence that melatonin shortens sleep latency, and induces faster sleep [17-25]. Improved sleep efficiency and to reduce intermittent wakefulness are also reported . Subjects tend to assess their sleep as ‘deeper’ after treatment . Taking it as a sleeping aid, no “hangover” effect (sleepiness, altered mood, and reaction time) is observed on the morning after treatments, as assessed with mood and performance tests administered [19,24]. However, it seems that exogenous melatonin exerts observed hypnotic effects only when circulating levels of endogenous melatonin are low [22,24]. Faster and smoother sleep onset is credited to melatonins significant hypothermic effect .
Available data clearly suggests that melatonin as supplement may be useful in treating insomnia as well as in treatment of other sleep disorders.
Melatonin supplements are safe in low doses for short-term and long-term use, however it does have some minor side effects which go away when you stop taking the supplement. Side effects may include: sleepiness, lower body temperature, vivid dreams, morning grogginess and small changes in blood pressure.
(Other common names: N-Acetyl-5-methoxytryptamine, Pineal Hormone, Melatonine, Circadin, Melatol, Melovine, Melapure, Melatonex)
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