Melatonin: Physiological effects

Last modified: Sunday, 31. May 2009 - 5:11 pm

Melatonin causes general effects that are felt throughout the body and specific effects on certain organs. Melatonin reduces body temperature so people may feel slightly cooler after they take a supplement containing this substance in addition to becoming sleepy. Melatonin produced by the pineal gland also has this effect. However, there is a slight variation in the ability of melatonin to reduce body temperature. Melatonin is not as effective at lowering body temperature during the luteal phase of the menstrual cycle, which is just before ovulation. Physicians and many women have long known that a woman’s body temperature increases slightly just before ovulation.
Melatonin also causes vasoconstriction, which is the narrowing of the blood vessels.
Sleep/circadian sleep rhythms
The most well-known effect of melatonin is that it regulates the sleep cycle. Production of melatonin by the pineal gland increases with darkness. Increased melatonin levels, in turn, promote sleep.
Melatonin is not essential for sleep to occur. Many people take naps during the daylight hours and can stay awake into the early hours of the morning, although melatonin levels are very low at this time. Still, most people find it very difficult to stay awake all night and sleep during the day. Melatonin does, somehow, make sleeping easier. The abnormal production of melatonin has been linked to sleeping difficulties. Many blind people who do not have any perception of light also have an abnormal pattern of melatonin production. They may not have higher levels during the night or their highest levels of melatonin can actually occur during the day. These individuals tend to have sleeping disorders. They may have difficulty sleeping at night and take naps during the daytime. Exactly how melatonin is related to the maintenance of a normal sleep cycle is not yet understood.
Reproductive effects
Melatonin influences the level of other hormones in the body. It has been shown to have a particularly strong effect on prolactin and luteinizing hormone, both of which are major reproductive hormones. Some authorities caution that it may reduce the sexual drive in men.
The natural production of melatonin by the pineal gland peaks at the age of only four or five years of age. Since a decrease in its production occurs during sexual maturation, researchers warn that taking it during adolescence may have a detrimental effect on puberty.
Immune effects
Melatonin has been shown to boost certain parts of the immune system. It boosts the activity of natural killer cells, a type of immune cell. It also prevents apoptosis, a type of destruction of T-lymphocytes, which are other important immune cells found in the bloodstream. Melatonin also limits the effect of corticosteroids on the immune system. Corticosteroids are very potent drugs that are sometimes used to inhibit the immune system. Some people have autoimmune diseases in which their immune system attacks the cells of their own bodies. Physicians frequently prescribe corticosteroids for these individuals.
Harmful side effects
As there has been very little research studying the safety of taking melatonin, most experts caution people against using it, or to limit their intake of it. However, it is known that melatonin may be particularly dangerous for certain people. People should not take melatonin if they:
• Have not finished, or even started, puberty. The decreased production of melatonin by the pineal gland that occurs during late childhood and early adolescence may be linked to sexual maturation.
• Are breast-feeding. Melatonin affects the level of prolactin, a hormone that controls the production of breast milk.
• Are pregnant. Melatonin influences the levels of various sex hormones.
• Have a history of stroke or other cerebrovascular disease. Melatonin causes vasoconstriction, which, under certain circumstances, can be a risk factor for cerebrovascular disease.
• Have a history of liver disease. Melatonin is metabolized in the liver. Melatonin may remain within the body for a longer time and in larger amounts if the liver is not able to break down this hormone so that it can be excreted.
• Have an autoimmune disease. Melatonin has been shown to boost some processes of the immune systern. People who have an autoimmune disease already have an overactive immune system, which is attacking their own body. Taking a substance such as melatonin, which further stimulates immune activity, may make their symptoms much more severe.
• Are depressed or have a neurological disorder. Although melatonin may be linked to the development of seasonal affective disorder (SAD), its exact role in emotional and neurological illnesses remains unclear.
• Take certain medications such as benzodiazepines, methamphetamines, dehydroepiandrosterone, magnesium, zinc, corticosteroids, and succinylcholine.
Melatonin can cause some adverse effects. People should contact their health care provider if they have difficulty sleeping or feel too sleepy, develop a headache, chills, or itching, feel confused, or have a fast pulse after taking melatonin. Individuals who will be undergoing surgery should tell the anesthesiologist that they have been taking melatonin.
Long-term health effects
No one knows what kind of effects may result after taking melatonin for a long period time. It has not been studied and its widespread use only started during the 1990s. Many experts strongly warn people not to take melatonin, or to at least limit their use to a short period of time such as for a few days after taking a long trip as a short-term treatment for jet lag. But some experts caution that withdrawal may occur after taking very high doses of melatonin.

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