Diurnality, Type 2 Diabetes, and Depressive-Like Behavior.

Although type 2 diabetes (T2DM) and depression are associated with disturbances in circadian rhythms, most studies of these diseases use nocturnal mice and rats while modeling diurnal humans. We suggest that the development of T2DM and depression are related to changes that accompany the switch from the mammalian ancestral nocturnal activity to the current diurnal one. We show that diurnal sand rats ( Psammomys obesus) held outdoors in laboratory cages (where they are exposed to natural environmental conditions) and fed a standard rodent diet do not develop T2DM in contrast to animals held indoors (where the only cycling environmental condition is light) fed the same diet. Moreover, keeping sand rats under a short photoperiod dampened behavioral and molecular daily rhythms, resulted in anxiety- and depressive-like behavior, and accelerated the development of T2DM. We suggest that the disturbed rhythms disrupt the internal temporal order and metabolic pathways controlled by feeding and the circadian system, resulting in the development of T2DM and depressive-like behavior. We further suggest that using nocturnal mice and rats as sole model animals may limit research, especially when studying circadian rhythm-related diseases.