sentences:  In "Drosophila", "per" mRNA levels oscillate with a period of approximately 24 hours, peaking during the early subjective night.The "per" product PER also oscillates with a nearly 24-hour period, peaking about six hours after "per" mRNA levels during the middle subjective night.When PER levels increase, the inhibition of "per" transcription increases, lowering the protein levels.