English: Pr converts to Pfr during the day time and Pfr slowly reverts back to Pr during the night time. When nights are short, an excess amount of Pfr remains in the day time and during long nights, most of the Pfr is reverted back to Pr.
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This media file needs to be cleaned up, because:The first sentence meant short "nights" instead of "lights". For help, see Commons:Media for cleanup.