Yes, the fluctuation of melatonin levels in the human body is an example of a periodic event. A periodic event is characterized by occurrences at regular intervals or cycles over time.
In this case, melatonin levels rise in the evening as the body prepares for sleep, typically in response to the decrease in light, and they peak during the night. As morning approaches and light increases, melatonin levels decrease, helping to promote wakefulness. This cycle of rising and falling melatonin levels correlates with the 24-hour day-night cycle, or circadian rhythm, which is regulated by the body's internal biological clock.
The predictability of this pattern—levels rising in the evening and falling in the morning—demonstrates that it occurs in a regular and repetitive manner, thus qualifying it as a periodic event.