Write one function of melatonin.

Explanation

### Step-by-Step Solution

1. **Understanding Melatonin**: Melatonin is a hormone produced by the pineal gland in the brain. It plays a crucial role in regulating various biological processes.

2. **Production and Regulation**: The production of melatonin is influenced by light exposure. The pineal gland is sensitive to light; when light hits the retina, melatonin production decreases. Conversely, during the night, when there is less light, melatonin production increases.

3. **Function of Melatonin**: One of the primary functions of melatonin is to regulate the sleep-wake cycle. It helps signal the body to prepare for sleep as darkness falls, promoting a sense of sleepiness.

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