Cities are wasting LEDs' ability to cut light pollution, with night-sky brightness rising 9.6% per year

Citizen-science data collected between 2011 and 2022 show the average night sky brightened by 9.6% per year - equivalent to doubling sky brightness every eight years^[1], even as cities worldwide swapped sodium streetlights for supposedly efficient LEDs. The technology that could have dimmed our nights is instead making them brighter, and the reasons are rooted in economics and policy rather than physics.

The rebound effect is erasing the energy savings

LEDs are genuinely more efficient than the high-pressure sodium (HPS) lamps they replace. The problem is what municipalities do with the savings. When Portuguese street-lighting data were analysed, the average lumen output of new LED installations had risen 120% compared with the pre-existing lighting they replaced - more than double the light for the same roads. Cheaper light has not produced less light; it has produced more.

The mechanism is straightforward: lower running costs free up budget, and that budget gets spent on extending lighting to previously unlit areas^[1]. Satellite analysis has confirmed the pattern repeatedly, finding that Earth's outbound radiance into space has continued to rise steadily despite the LED rollout. Researchers describe this as a classic rebound effect - the same dynamic that leads fuel-efficient car owners to drive more miles.

Spectrum matters as much as quantity

The shift from narrow-spectrum HPS lamps to broad-spectrum white LEDs has introduced a second problem: colour temperature. Early cost-driven LED street-lighting rollouts commonly used fixtures in the 4,000-5,000 K range, emitting significantly more short-wavelength blue light than the amber glow they replaced. Blue light scatters more readily in the atmosphere, amplifying skyglow beyond what lumen counts alone would predict.

The biological consequences compound the astronomical ones. Research published in April 2025 found that:

• Blue LED exposure (464 nm) sustained melatonin suppression at 7.5 pg/mL after two hours of evening exposure
• Red light over the same period allowed melatonin to recover to 26.0 pg/mL
• The suppression effect was strongest in younger participants and men

Studies on night workers have linked artificial light at night to sleep disorders, cardiovascular disease, type 2 diabetes, obesity, and depression. Outdoor lighting that floods into bedrooms and disrupts circadian rhythms extends those risks to the general population.

What the technology actually allows

The frustration is that LEDs are, in principle, the right tool for the job. Modern fixtures are controllable, dimmable, and spectrally tunable in ways that sodium lamps never were. Tunable LED lamps have the potential to produce significant reductions in melatonin suppression from room illumination - and the same logic applies outdoors. Several cities that installed cool-white LEDs in the 2010s have already been forced to retrofit warmer 3,000 K alternatives once the skyglow increase became undeniable^[1].

Newcastle University researchers published work in February 2026 showing that small reductions in brightness during twilight, better shielding to limit skyglow, and adjusting LED spectra to limit short-wavelength emissions could reduce ecological harm without compromising human safety. The tools exist. The gap is in procurement standards and political will.

The question for the next municipal lighting cycle is whether engineers and city planners will write specifications that exploit LEDs' tunability and controllability - or whether the rebound effect will simply continue to fill every dark corner with cheap lumens.