MTNR1B is a critical mediator in melatonin signaling that links circadian rhythms to T2DM, and a common variant of MTNR1B rs10830963 seems to further alter the nocturnal release of melatonin, inhibiting its release from β-cells; this alteration leads to a delayed rhythm and elevated glucose levels during the night as well as in the early morning. This evidence concerns the gene MTNR1B and type 2 diabetes mellitus.