A 35-year-old pilot presents to her physician with complaints of difficulty falling asleep after long-haul flights. She reports frequent international travel and trouble adjusting to different time zones. She prefers to avoid prescription medications and asks if there is a natural supplement that could help regulate her sleep-wake cycle. The physician recommends an over-the-counter supplement that is naturally produced in the body and helps regulate circadian rhythms. Where in the body is this substance primarily produced?
