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How do light signals from the retina affect the circadian rhythm?

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The circadian rhythm is regulated by the hypothalamus, specifically a region called the suprachiasmatic nucleus (SCN), which acts as the body's master center for integrating light signals to establish sleep patterns.12

When the retina detects light, special photoreceptor cells transmit signals to the SCN via the retinohypothalamic tract. These signals are then relayed to the pineal gland, which is responsible for producing the hormone melatonin.23

The interaction functions as follows:
* In the presence of light: Melatonin production is inhibited, which promotes wakefulness and alertness.23
* In the absence of light: Melatonin production increases, signaling the body to prepare for sleep by inducing physiological changes such as decreased body temperature and respiration rate.23

By modulating melatonin levels based on environmental light, the SCN aligns biological processes—including hormone release and body temperature—with the external day-night cycle.12