TWILIGHT TRANSITIONS PROMOTE CIRCADIAN ENTRAINMENT TO LENGTHENING LIGHT-DARK CYCLES

The upper limits of entrainment of the circadian activity rhythm were compared in hamsters initially exposed to daily light-dark (LD) cycles with either abrupt (LD-rectangular) or simulated twilight (LD-twiligh t) transitions. Daytime illuminance (10 1x) and the total amount of li ght emitted per day were the same under the two LD cycles. One-half of the animals in each condition had access to dark nest boxes. The peri od of the LD cycles was then increased from 24 to 26 h, by 5 min per d ay. All animals in LD-twilight remained entrained to the lengthening c ycle, whereas 60% of those in LD-rectangular began to free run well be fore the period of the cycle reached 26 h. These effects were independ ent of nest box availability. The lengthening LD cycles exerted clear aftereffects on the period of the rhythms in constant darkness, the ma gnitude of which was related to the efficacy of prior entrainment. The results indicate that twilight transitions raise the upper limit of e ntrainment to LD cycles, suggesting that their inclusion increases the strength of the LD zeitgeber.