It affects bird migration, results in algae blooms when near lakes, harms navigational instincts for species including sea turtles, and negatively impacts nocturnal species. There’s a good summary here: https://en.m.wikipedia.org/wiki/Ecological_light_pollution

Everything people mentioned is true. Algae blooms, insect disruption, confusing newly hatched baby turtles, all sorts of impacts on human health, etc. But one thing often left out of the conversation around light pollution is the impact it has on plants. Many plants heavily depend on the photoperiod to tell what time of the year it is and when it should bloom. Similarly to insects, the impacts it has on plants can have a lot of downstream effects on the entire food web

Ultimately, we'll never probably be able to measure the full range of impacts it has though. It's a really complex issue

The effects of artificial light are manifold. Assigning upsides and downsides is left as an exercise.

https://pubmed.ncbi.nlm.nih.gov/?term=light+pollution+human

Also note humans seem to have an inherent "light budget" the cheaper it is, the more we use.

https://duckduckgo.com/?q=historic+light+cost+v.s.+use&t=ffs...

artificial lights do mess with wildlife a lot. https://www.darksky.org/light-pollution/wildlife/

Many animals use light or the absence of it as a guide to do various things, so when we artificially extend the day cycle with night pollution it disrupts them in a myriad of ways.

It disrupts insects (and thus, everything downstream of them in the food chain):

https://www.sciencedaily.com/releases/2021/03/210317141651.h...

It's a nuisance for sleeping. It appears to be bad for bugs, birds, nocturnal animals in general. It's certainly bad for sea turtles. I'm not sure what else, but man... this is one of my pet peeves. Almost every neighbor in my neighborhood has some kind of outdoor lighting that they leave on all night. I simply don't understand it, and it annoys me any time I'm out in the back yard around the firepit, looking up at the stars.

Also effects human sleep cycles.