Exogenous evening melatonin mitigates the negative impact of night-time light pollution on sleep and metabolism in both sexes of diurnal zebra finches

Artificial dim light at night (dLAN) depresses nocturnal melatonin peak, along with disruption in sleep and metabolism. This study asked whether exogenous melatonin administration could reverse the effects of dim light at night on sleep and metabolism in a diurnal songbird. To answer this, three groups of zebra finches (Taeniopygia guttata) of both sexes (n=6/group/sex) hatched and raised in our indoor aviary were used. While a group remained on the dark night (LD control; 12L:12D, L=150 lux: D=0 lux), as before, to the other two groups the 12-h dark night was replaced with 5-lux (equivalent to 0.048 W/m²) dim light (12L:12dLAN; dLAN). Half an hour before dark onset, consecutively for the next 11 days, the birds of a dLAN group received subcutaneous injections of melatonin (10 µg/ 100 µl vehicle), while those of the other dLAN and LD groups (controls) received 100 µl vehicle alone. Exogenous melatonin mitigated dLAN-induced disruptive effects on both nocturnal sleep and metabolism. As compared to LD and vehicle-treated controls, melatonin-treated birds showed a significantly reduced nocturnal sleep frequency (fewer awakenings) and longer sleep bout as well as reduced night-time feeding and body fattening, and restoration of normal hepatic expression of genes associated with glucose and lipid metabolism. In particular, the elevated melatonin levels prevented the midnight increase in g6pc, irs1, fasn, star and egr1, and augmented the sirt1 mRNA levels under the dLAN environment. These results suggest that nocturnal melatonin levels are a crucial component of the regulatory pathways underlying nocturnal sleep and metabolism, independent of sex, in a diurnal songbird species.