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How does darkness affect melatonin production by the pineal gland?

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In humans, the production of melatonin by the pineal gland is directly regulated by environmental light-dark cycles. This process functions through a "photoendocrine system" that links the retina to the brain.12

When the retina detects light, it sends signals to the suprachiasmatic nucleus (SCN) in the hypothalamus, which in turn inhibits the pineal gland’s production of melatonin. Conversely, in the absence of light, this inhibition is lifted, and the pineal gland is stimulated to produce and secrete melatonin.12

The biosynthesis of melatonin involves the activation of the enzyme serotonin N-acetyltransferase (AANAT) during these dark periods. Consequently, melatonin levels typically peak during the nighttime hours. This nocturnal rise in melatonin serves as a biological signal that helps entrain circadian rhythm and promotes sleep by inducing physiological changes, such as a decrease in body temperature and respiration rate.21