Tasimelteon-d5 is the deuterium labeled Tasimelteon. Tasimelteon is an orally active and selective dual melatonin receptor agonist (DMRA). Tasimelteon has 2.1-4.4 times greater affinity for the MT2 receptor than for the MT1 receptor. Tasimelteon is a circadian regulator and has the potential for Non-24-Hour Sleep-Wake Disorder (Non-24).