Peak Secretion Time of Melatonin Hormone

Peak secretion of melatonin hormone occurs mainly during:

Correct Answer: Midnight

Melatonin is a hormone secreted by the pineal gland (smallest endocrine gland of the body). Its secretion follows a strict circadian (24-hour) rhythm: levels begin rising in the evening after darkness, peak at midnight (2:00–4:00 AM), and fall sharply in the early morning with exposure to light.

Key Facts About Melatonin

• Precursor: Tryptophan → Serotonin → Melatonin.
• Trigger: Darkness stimulates; light suppresses melatonin secretion (via retinohypothalamic tract → suprachiasmatic nucleus → pineal gland).
• Functions:
  • Regulates circadian rhythms and sleep-wake cycles.
  • Regulates seasonal reproductive cycles (short-day breeders like sheep, deer are stimulated by long melatonin peaks in winter).
  • Acts as an antioxidant.
• Seasonal breeding: Melatonin signals long nights → stimulates breeding in short-day breeders (sheep, goats) and inhibits breeding in long-day breeders (horses).

Why Other Options Are Wrong

• Early morning → Melatonin levels are declining as light begins; secretion is at its lowest during daytime.
• Afternoon → Melatonin is suppressed by daylight; near-zero levels in the afternoon.
• Late evening → Secretion begins rising at late evening but has not yet reached its peak.