OPN4 and sleep disorder: Our review revealed that some SNPs of the OPN4 gene, such as P10L, I394T, and R168C, are associated with affective states, changes in chronotype, and sleep disorders: P10L variant has been associated to seasonal affective disorder (SAD), chronotype, and chronic insomnia; I394T variant has been linked to the pupillary light response (PLR) and sleep/wake timing, while R168C variant has been associated with delayed sleep-wake phase disorder (DSWPD).