I'm going to listen in to a talk by Prof. James Lowenthal from Smith College called "Darkness at Night: Essential for Health", for the IAU-CPS community education hub. I'll share some highlights (there aren't very many people here listening, which is crappy because James is such an excellent teacher!)
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The difference in brightness between day and night used to be a factor of ~a million, now it's a factor of less than 1,000 because of light pollution and spending time indoors in daylight.
Cardiovascular disease, cancer, diabetes, obesity, sleep disruption, hormone imbalance, mood disorders, are all demonstrated to be caused or exacerbated by exposure to artificial light at night. Blue light is particularly bad.
Only 20 years ago, scientists discovered we have a 3rd type of non-vision-related light receptors in our eyes: Intrinsically photosensitive retinal ganglion cells (what a mouthful... err... eyeful?) We also have a "chemical clock" in every cell nucleus that controls our circadian rhythm (2017 Nobel prize for this discovery!)
Robust circadian rhythms are important for good health. Exposure to blue light at night disrupts our circadian rhythms.
White LEDs are particularly bad with a "blue spike" that can suppress melatonin and disrupt our circadian rhythms. American Medical Association recommends against using blue-heavy LEDs.
Most vertebrates affected by this.
Everyone wants to see the stars. Astrotourism is booming! Stargazing is a way to connect with our ancestors. There are well-documented, peer-reviewed health benefits to experiencing awe: reducing anxiety and stress, and one of the most reliable ways to experience awe is by looking at unpolluted night sky.
Light pollution is unwanted light: not being actively used, too much, poorly aimed.