The lower entrainable limit of rat circadian rhythm to sinusoidal light intensity cycles: A preliminary study

The lower entrainable limit of the circadian behavioral rhythm was examined in rats exposed to sinusoidal light intensity cycles with maximum illumina nce of 20 lux and the minimum of 0.01 lux. The period (T) of the light inte nsity cycle was initially kept at 23.5 h for 20 cycles, and then shortened to 23 h for 33 cycles. Thereafter the rats were released into constant dark ness. Five out of 10 rats entrained their circadian rhythms to T = 23.5-h c ycle, and they also entrained to the T = 23-h cycle. The phase angle of ent rainment was almost unchanged when T was shortened from 23.5h to 23h. These results suggest that the T=23-h cycle was close to the lower limit of entr ainment.