The rat pineal N-acetyltransferase rhythm persists after a five-hour, but disappears temporarily after a seven-hour advance of the light-dark cycle: a six-hour shift may be a turning point.

After a 5-h advance of a lighting regime with 12 h of light and 12 h of darkness by shortening of one dark period, the rat pineal N-acetyltransferase (NAT) rhythm was retained, though at the beginning in a changed waveform, and re-entrained to the shift within 5 to 6 cycles. After a 7-h advance of the regime, the NAT rhythm faded away between the 2nd and 4th cycle and re-entrained to the shift within 7 cycles. A 6-h advance of the lighting regime might thus be a turning point for the persistence or temporal disappearance of the NAT rhythm.