Too little darkness: How artificial light at night is harming humans and wildlife

For most of human history, night meant darkness. Today, that darkness is disappearing beneath streetlights, illuminated buildings, advertising hoardings, vehicles and increasingly bright urban skylines.

The glow of cities is often seen as a symbol of progress. But as streets, buildings, billboards and homes remain illuminated deep into the night, scientists are increasingly warning that the loss of darkness is becoming an environmental problem in its own right.

The problem is known as light pollution, or artificial light at night (ALAN), and scientists are increasingly treating it as more than an inconvenience for astronomers. Research shows that excessive or poorly directed artificial light can interfere with biological rhythms, alter animal behaviour and affect human health.

Unlike air or water pollution, light does not leave a physical residue in the environment. Yet it can alter the natural cycle of light and darkness that organisms have evolved around for millions of years.

When night stops being night 

One of the biggest concerns is the disruption of the body's circadian rhythm — the internal clock that regulates sleep, hormones and other biological processes.

Exposure to artificial light at night can suppress melatonin, the hormone associated with sleep, and interfere with normal circadian timing. Reviews of existing research have linked artificial night-time light with a range of potential health effects, although researchers continue to investigate the strength and causes of these associations.

The colour of the light can matter too. Blue-rich light is particularly effective at affecting the biological systems that respond to darkness, making the widespread use of bright white LEDs an important consideration in discussions around night-time lighting.

Wildlife faces a different kind of night 

For animals, the consequences can be immediate. Insects may be drawn towards artificial lights, disrupting feeding, reproduction and movement. Birds can alter their activity and navigation patterns, while nocturnal mammals may avoid illuminated areas altogether. Artificial light can also change predator-prey relationships and fragment habitats that would otherwise remain connected after sunset.

Recent research is adding to that picture. A 2026 meta-analysis examining 675 effect sizes from 36 studies across 30 bird species found that artificial light at night consistently affects avian physiology and behaviour, although the direction and magnitude of the effects vary among species.

The impact does not stop with animals. Plants also respond to changes in the timing and duration of light exposure, potentially altering growth, flowering and other biological processes.

The ecosystem problem 

The larger concern is what happens when these individual disruptions interact.

Artificial night-time light can change when organisms feed, reproduce, migrate or become active. That can affect interactions between species and ultimately alter the structure and functioning of ecosystems. Research published in Nature Climate Change has also found that artificial light can influence ecosystem metabolism, highlighting that its effects extend beyond individual animals.

The problem is growing alongside urbanisation. Artificial light at night is now widespread, while newer research continues to identify species and ecosystems whose responses remain poorly understood.

The goal is not to eliminate lighting. Artificial light is essential for transport, public spaces, workplaces and safety. The challenge is using the right amount of light, in the right place, for the right amount of time.