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The 24 hour (diurnal) rhythm of our body such as the sleep-wake cycle is regulated by the hormone:

1. calcitonin
2. prolactin
3. adrenaline
4. melatonin

Correct answer: melatonin

Solution

Melatonin is secreted by the pineal gland and helps regulate circadian (24-hour) rhythms, especially the sleep-wake cycle. The other options have different roles: calcitonin regulates calcium, prolactin supports milk production, and adrenaline mediates stress responses.

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