We know that when artificial light shines directly from a light source onto living things at night, it affects their behavior and physiology. But what about the diffuse glow of the night sky, does that affect the environment?
Turns out it does. Read more in this brief thread 🧵
#Skyglow image by @andreas_jechow (1/17)
Before getting to skyglow, let's just quickly remind ourselves that light at night is one of the most dramatic environmental changes that humans have made to the Earth. Over many parts of the Earth, true night no longer occurs, we experience only twilight or daytime.
The image shows the brightness of skyglow around the world: https://www.science.org/doi/full/10.1126/sciadv.1600377 (2/17)
For every place colored yellow, the sky directly above on a starry night is at least twice as bright as natural.
But does shining a bit of light really change behavior? And what about physiology?
Well, changes in behavior are simple to demonstrate: the whole reason we use artificial light at all is to allow us to see better at night, in order to enjoy ourselves and enable the nighttime economy.
Just as light allows people to get around and stay up later, animals can also use it for the same purpose. This can be beneficial or detrimental, depending on the animal. But it's clearly a huge change compared to their evolutionary experience. (3/17)
This is my favorite photo for demonstrating the impact of #LightPollution on physiology. The image was taken by shows a soybean field illuminated by a badly directed streetlight.
What's happening here is that soybeans are supposed to grow leaves in the early part of summer, and as nights get longer, they should make #soybeans and turn brown. In the green area, the plants don't understand what time of year it is, and it's therefore a complete loss for the farmer.
The reason I love the photo so much is because you can see the shadow of the light mast on the field.
The photo was taken by Dwaine Eddie McGriff & Ben Tankersley, and originally posted to Xitter (the post no longer exists).
The problem could be entirely solved by using a streetlight with strong backlight shielding (i.e. shining the light only on the roadway).
(4/17)
Plants and animals are different [citation needed], but we've both evolved on the same Earth. So the physiology of animals is also affected by exposure to #LightPollution, and this extends to humans and #HumanHealth.
We have the problem that we spend most of the day indoors under artificial lights (much darker than natural day), and then spend the evening indoors under artificial lights (much brighter than natural night). Especially for night owls, this lack of a clear day/night signal screws up our internal #Circadian clocks. These clocks control things like metabolism, so messing with them causes #health problems: https://doi.org/10.1177/23727322231193967 (5/17)
Ok, but what about skyglow, the diffuse glow in the sky created by all of the lights we use acting together. Does that really have an impact on the environment?
Image by Zoltán Kolláth: https://lossofthenight.blogspot.com/2015/03/effect-of-single-floodlamp-in-natural.html (6/17)
The quick answer is "it surely must": https://doi.org/10.1007/s10980-013-9936-3
After all, many organisms change their behavior in response to changes in moonlight or cloudiness. This was also true for human beings, before we had artificial light, as demonstrated by these clips from the novel "Tess of the d'Urbervilles" written by Thomas Hardy in 1891. (7/17)
But the trouble is, how do you do a controlled experiment to show that skyglow affects an animal, plant, or biodiversity? It's extremely difficult, because it's hard to set up a natural and artificial condition in the lab, and you can't really ask a huge city like Berlin to just turn off all the lights for a few hours, because you want to see how the mice in the fields react.
(An additional difficulty is that biologists are nervous about the possibility of null results, because they are less likely to promote your career. That's a whole other problem, but it is an additional factor that prevents such experiments from being done.)
(8/17)
First up, "Skyglow relieves a crepuscular bird from visual constraints on being active" by Evens et al.: https://doi.org/10.1016/j.scitotenv.2023.165760
In the paper, they show that #nightjar birds behave differently in locations with or without skyglow, and importantly that the relation with cloud cover is opposite between the two locations. (10/17)
This paper has just been followed up by a new one titled "Skyglow facilitates prey detection in a crepuscular insectivore: Distant light sources create bright skies": https://doi.org/10.1016/j.envpol.2025.125821
They show that nightjars use the glow of the sky as a background for detecting prey. (11/17)
Back to the environment...
Just a few days ago, my colleague Jeremy Fonvielle published a paper for our Illuminating Lake Ecosystem experiment, in which he shows that our diffuse simulation of skyglow increases the abundance of cyanobacteria, and changes the way organic matter cycles in lakes: https://doi.org/10.1016/j.watres.2025.123315 (13/17)
The first is "Artificial light at night decreases plant diversity and performance in experimental grassland communities" by Bucher et al.: https://doi.org/10.1098/rstb.2022.0358
Here they showed that both growth and species diversity was negatively affected by the addition of #LightPollution, and that the impact starts at skyglow-like light levels. (15/17)
Thanks for reading along! I see that there have been a lot of notifications while I was doing the thread - I'm glad that you found it interesting!
Update next day: thanks to everyone who boosted, and welcome to everyone who has newly followed me! (17/17)
@skyglowberlin In case you're interested in chatting this thread so I can trace all those side comments ...
Calling @Chartodon spine...
@skyglowberlin Sorry, auto-carrots... That should read ...
In case you're interested I'm *charting* this thread ...
@skyglowberlin I got the invocation wrong and it didn't "spine" the chart, so I'm doing it again:
Calling @Chartodon spine ...
I need to look at the 'bot to make sure it's a little cleverer with the parsing.
@skyglowberlin
Reminds me of them turning off memorial lights of the trade center during migratory season to allow them to progressing on their migration.
https://www.sierraclub.org/sierra/birds-scientists-and-9-11-memorial
A few years back I was frustrated that there was no "Rare Ant Conservation Society" undeterred, I started researching what existing conservation efforts would have the biggest impact on less common ants. I came up with two in the US: The Xerces Society and? Dark Sky International.
It turns out that light pollution is a big deal for insect diversity. One simple way to help insects is to simply reduce it. Dark Sky mostly plays this up as benefiting birds, and that's nice too.
yeah yeah yeah songbirds whatever, tell me more about the bugs.
@futurebird As a child, I used to see fireflies in summer in the village I grew up in.
I don't think my children have ever seen any and I'm mad about that and want to do something, but I don't know what.
@Landa @futurebird Politicians and city lighting departments are surprisingly responsive to comments from the public (at least in some cities). Speaking to them about why it is important to you, and what your vision for lighting in the future is can help.
I'm not an expert on fireflies, but things that are likely to help are:
o Avoid blue light (use low CCT or amber light)
o Light according to the lowest appropriate lighting class for the street
o Avoid shining lights on green areas (direct the lights to hit only the necessary areas)
o Allow dead leaves to accumulate under bushes
o Cut grass infrequently, allow it to get quite tall before trimming
Christopher Kyba 🇨🇦🇪🇺
Grah am Jan z
Darrin West
David Ruffner
glasspshr
Colin the Mathmo
Sarah W
John-Mark Gurney
Jon Sullivan
myrmepropagandist
Landa 
Ben Rosengart
John Dal
KChernecky For Democracy
NKT
radioscout
acm