Melatonin patterns in quail vary with day length and internal body clocks

Melatonin levels in the eyes, pineal, and blood of Japanese quail showed a robust daily rhythm with higher levels occurring at night.

• Melatonin amplitude was depressed and its duration lengthened in quail under a 6:18 light-dark cycle compared to a 16:8 cycle.
• Blood melatonin levels mirrored the rhythms observed in the pineal gland and eyes, suggesting a relationship between these tissues.
• The ocular melatonin rhythm persisted independently of optic nerve connections and could reentrain to changes in the light-dark cycle.
• The ocular rhythm lasted for at least 2 days in constant darkness, indicating a potential intrinsic circadian clock within the eyes.

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