RATE OF RE-ENTRAINMENT OF CIRCADIAN-RHYTHMS TO ADVANCES OF LIGHT-DARK CYCLES MAY DEPEND ON WAYS OF SHIFTING THE CYCLES

When an 8-h advance of a light-dark (LD) cycle was accomplished by shortening of one dark period by 8 h, the rat pineal N-acetyltransferase rhythm was abolished during the first 3 subsequent cycles and reappeared in its original waveform during the fifth cycle only. When the 8-h advance shift was accomplished by lengthening of two consecutive light periods by 8 h, the N-acetyltransferase rhythm persisted and attained its original waveform by 2 days earlier than under the former shift. Re-entrainment is thus more rapid when the advance shift is accomplished by twice delaying than by once advancing of the LD cycle by 8 h. © 1990.