Saving from the dark side of light at night: differential effects of complete darkness in the first and second half of dimly illuminated nights on sleep and metabolism.

Exposure to dim light at night (dLAN) affects circadian rhythms and disrupts sleep and metabolism. However, given differential light sensitivity of circadian rhythms during early and late hours of the night, dLAN may have dissimilar detrimental effects in two halves of the night. If so, the disruptive effects of dLAN would be mitigated at different levels if one of the two halves of the night were replaced by complete darkness. We investigated this using both sexes of diurnal zebra finches that were exposed for three weeks to 12 h light (~ 150 lx) coupled with 12 h dark (0 lx), dLAN (5 lx), and half-dark and half-dLAN (6 h dark + 6 h dLAN, or vice versa) nights. dLAN disrupted the nocturnal sleep (shorter sleep bouts, reduced nocturnal sleep with frequent awakenings), and impaired glucose and fat metabolism as evidenced by body fattening and concurrent increase in g6pc, irs1 and star, and decrease in glut5 and sirt1 gene expressions in the liver. The substitution of half of dLAN with complete darkness mitigated much of the negative effects, with a much better alleviative response when the imposed darkness period covered the first half of the 12 h 'dLAN' night. Notably, the mitigation effects of darkness period were sex dependent. These results provide insights into differential temporal sensitivity of the night to negative impacts of the emerging 'light pollution' threat in an over-lit urban environment.