A modeling approach shows the effects of different light–dark schemes on the entrainment ability of the suprachiasmatic nucleus

In mammals, an endogenous clock located in the suprachiasmatic nucleus (SCN), synchronizes physiological and biological rhythms to the environmental light–dark cycle. In experiments, most researchers applied rectangular scheme as the external light–dark scheme received by the SCN neuronal oscillators. However, the external light intensity changes gradually throughout the day. Therefore, trapezoidal schemes (twilight) or sinusoidal schemes were also applied. Thus far, the effects of different light–dark schemes on the oscillators of the SCN did not get fully explored. In the present study, we theoretically analyzed how the five common light–dark schemes affect the entrainment ability of the SCN, based on a Poincaré model. We numerically found that when the maximum light intensity, the minimum light intensity, and the total amount of light exposure per cycle were the same, the largest entrainment range was obtained in the oscillators receiving more light in the daytime. However if, under the condition of 12:12-h illumination, the total amount of light exposure per cycle was the same, the maximum light intensity during the day leaded to an increased range of entrainment. Moreover, the entrainment range was reduced when the photoperiod was extended. Note that, increasing the maximum light intensity increased the entrainment ability of all light–dark schemes. Our results exposes the important role of light–dark schemes in the entrainment ability of the SCN network, and provides a potential explanation for the diversity of the entrainment range between diurnal and nocturnal animals.