Photorefractoriness in Mammals: Dissociating a Seasonal Timer from the Circadian-Based Photoperiod Response

Soay sheep exposed to summer light for 6 or 30 weeks showed changes in hormonal cycles but not in circadian gene expression.

• Prolonged exposure to constant light conditions induced photorefractoriness in Soay sheep.
• The expression of clock genes in the brain did not change with the animals' altered physiology.
• Levels of specific hormones decreased in photorefractory sheep, resembling those seen in shorter day conditions.
• Findings suggest a distinction between circadian rhythms influenced by light and long-term endocrine cycles.
• The study indicates that two different timing mechanisms may operate in these animals, one for short-term responses and another for seasonal changes.

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