An experiment carried out at 01:30 every morning for 10 nights has revealed the main sources of artificial light polluting the night sky.
The city of Tucson, in Arizona, US, dimmed its 14,000 streetlights over that period.
"We used a satellite to measure what fraction of the total light emissions are due to the streetlights," explained physicist Dr Christopher Kyba.
Artificial light has been shown to affect our sleep and our health.
"And late at night, when people are sleeping - that is exactly when we can save a lot of energy," Dr Kyba, who is based at the German Research Centre for Geosciences in Potsdam, told BBC News.
His light-from-space experiment, published in the journal Lighting Research & Technology, showed that most of the artificial light wasted - by being sent upwards into space, rather illuminating its targeted sign, street or building on Earth - does not actually come from streetlights, but from other sources.
Advertisements, floodlights, lit buildings, facade lighting, parking lots and sports stadia - these are the types of installations responsible for most light emissions, Dr Kyba explained.
"That's really important information for policy makers and light pollution activists," he said.
"This does make it more difficult to solve, because there are so many contributors. It means everyone has to get together to decide what lights need to be lit at night, and how brightly."
Wasting energy while we sleep
Because emissions come from so many sources, the exact amount of energy wasted on inefficient or unnecessary artificial light is difficult to estimate. But the International Dark Sky Association estimates that 35% of artificial light is wasted by being poorly aimed or unshielded. This equates, in the US alone, to about $3 billion per year spent on "making the sky glow".
That familiar urban glow means that most people on Earth never see a naturally dark night sky, but it also affects migrating birds, insects and other animals, by disrupting the light-dark cycle they are tuned into.
A little bit of clever lighting
One recent project with a floodlit church in Slovenia - a country that passed a law to curb increasing light levels - revealed how relatively straightforward the problem is to solve.
A lighting company re-lit a church in a way that reduced the power consumption for the light by 96% (from 1.6 kW to only 58 W). It also reduced the total waste light emission by using simple masks to shine the light just directly on the church.
"A lot of people talk about climate emergency but never talk about light pollution," said Dr Kyba. "But it's an important part. And at night, when most of us are asleep, all that electricity could be going to do other things - charging electric vehicles, for example.
"It's the kind of thing that can be done with a little bit of cleverness and the will to take action."
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