Switching to light-emitting diodes (LEDs) was expected to reduce electricity bills for towns and make the night sky darker. Instead, in many areas, the opposite has happened. Municipalities have used the money saved from more efficient lighting to install more lights, keep them on longer, or increase their brightness. The result? In some places, the glow above cities has not decreased—it has actually grown. This unexpected outcome highlights a financial trap: the energy savings from LEDs were used to illuminate more rather than to save. The original reasoning behind switching to LEDs was sound. Compared to older sodium vapor lamps, LEDs use significantly less electricity, which means lower costs, less carbon dioxide emissions, and less light pollution. In 2023, 40% of street lamps in France were converted to LEDs, resulting in a 29% reduction in electricity use and less light pollution nationally. However, the national average hides a more complex picture. Researchers have documented a "rebound effect" where the cost savings from LEDs have been used to install more lights or increase their brightness, leading to more light being emitted into the atmosphere. One key reason for the increased brightness is the color of LED light. Unlike the yellow-orange light from older lamps, which is less scattered by the atmosphere, LEDs emit a white light rich in blue. This blue light is scattered more widely in the atmosphere due to a physical phenomenon called Rayleigh scattering, which affects short wavelengths more strongly. A study published in 2018 found that LED lights with a cool, blue-rich spectrum can scatter up to 2.5 times more light into the night sky than traditional lamps. This has worsened light pollution, making it harder for astronomers to observe the stars and disrupting wildlife that relies on natural darkness. The ecological impact is significant. In France, light pollution has increased by at least 49% between 1992 and 2017, and it continues to grow by 7 to 10% each year. About 85% of the territory is affected by light pollution, which disrupts nocturnal animals like bats. Even a small amount of artificial light can disrupt their behavior, forcing them to abandon roosts or reduce their hunting time. This unintended consequence has led some local authorities to implement solutions like turning off lights at night or reducing their brightness. Legislative changes are also underway, with proposals to limit the color temperature of LED lights to reduce blue light emissions while maintaining energy efficiency. These adjustments could help align the benefits of LED technology with the goal of preserving the night sky.