![]() ![]() ![]() ![]() Melatonin production also can be suppressed by light exposure, so this can be a factor affecting the circulating levels and sleep patterns. Normally, plasma melatonin levels are very low during the daytime and gradually rise in the evening as bedtime approaches, typically about 2 hours before a person’s natural sleep onset time however, the actual clock time of the melatonin rise can vary considerably among individuals. Although present in a variety of tissues, circulating melatonin is predominantly produced in the pineal gland under the control of the master circadian clock transcription-translation loops in the hypothalamic suprachiasmatic nucleus (SCN), which is entrained by exposure to the light-dark cycle. It is useful to think of our endogenous melatonin in 2 ways: 1) reactions occurring intracellularly that do not necessarily involve specific melatonin receptors and 2) the more familiar circadian role associated with regular changes in circulating melatonin and effects on melatonin receptors. 2 We are not conscious of most melatonin actions, but certainly experience its effects on sleep and waking. 1 As humans, we benefit from our endogenous melatonin in support of our complex cellular activity and in helping to orchestrate wide-ranging physiological processes influenced by our circadian system. Evolutionarily, it dates back billions of years serving essential cellular processes with its antioxidant and free radical scavenging functions, and more recently as a key element in the regulation of circadian rhythms and the sleep-wake cycle. It is present in nearly all life forms, including plants and animals, and even single cell organisms. ![]()
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