Adjustment of the rat pineal N-acetyltransferase rhythm to change from long to short photoperiod depends on the direction of the extension of the dark period.

Extension of the rat pineal N-acetyltransferase rhythm after transition from LD 16:8 to LD 8:16 proceeded more rapidly when the transition was accomplished by prolongation of the dark period into the morning than into the evening hours. After symmetrical or evening prolongation of the dark period, adjustment of the rhythm to the LD 8:16 regimen was almost completed within 6 days. After prolongation of darkness into the morning hours, a temporal overextension of the rhythm occurred.