Significance: In this work, ab initio multiconfigurational quantum chemical computations are used to understand how light sensitivity is controlled in biological photoreceptors. The authors focus on melanopsin: the recently discovered nonvisual retina pigment responsible for the regulation of unconscious visual reflexes and the synchronization of endogenous physiological responses to the dawn/dusk cycle (circadian rhythms). The investigation indicates that the light-induced isomerization of melanopsin is up to several tens of femtoseconds faster than the analogue isomerization of invertebrate and vertebrate visual pigments. It is also revealed that its thermal isomerization is controlled by an energy barrier higher than the barrier of dim-light visual pigments. These properties support the hypothesis of an extreme light sensitivity of melanopsins.