The Manhattan skyline — glimmering, grand but not always environmentally efficient — may need to go darker to go green.

Amid a far-reaching push to reduce New York’s environmental footprint, city officials on Wednesday weighed a City Council bill to limit internal and external light use in many commercial buildings when empty at night, a change that could affect some 40,000 structures and rethink the shape, or at least the hue, of what residents see when they look up.

The environmental considerations are clear: reducing potentially wasteful energy use as part of the city’s bid to curb its greenhouse gas emissions. The administration of Mayor Bill de Blasio has expressed support for passing a version of the bill, calling light pollution a citywide scourge for migratory birds and sedentary New Yorkers.

The hearing, accordingly, cast a wide net, touching on amphibious mating activity, the astrophysicist Neil deGrasse Tyson and even the very definition of nighttime. And it hinted at a host of complications in the proposed legislation, among them an exception for buildings found to be “a significant part of the city’s skyline.”