Final answer:
When light hits the retina in the morning, melatonin production is reduced, which promotes wakefulness and resets the circadian rhythm. This is crucial for regulating sleep-wake cycles and maintaining overall health. The correct option is (d)
Step-by-step explanation:
When the retina of the eyes detects light in the morning, it sets off a cascade of events involving the suprachiasmatic nucleus (SCN) in the hypothalamus and ultimately leads to changes in hormone levels in the body. One of the main effects is that melatonin production is inhibited. This occurs because light stimulation of the retina sends signals via the SCN to the pineal gland, telling it to decrease the synthesis of melatonin. Hence, there is a reduction in melatonin levels, which promotes alertness and wakefulness.
Furthermore, light exposure in the morning helps to reset our circadian rhythm. The circadian rhythm is like an internal clock that guides our sleep-wake cycle and other daily physiological processes. Exposure to natural light in the morning is crucial for maintaining a healthy circadian rhythm, which is why it's important to get exposure to sunlight early in the day when possible.
Based on this information, the correct answer to the question is that when the retina of the eyes detect light in the morning, the circadian rhythm is reset (d). This process helps regulate our sleep-wake cycles and has various health implications.