Colours of the Night: Spectrum-Specific Impacts of Light Pollution on Biota

Artificial light at night (ALAN) disrupts ecosystems by altering natural light cycles and affecting the physiology and behaviour of species, and represents a widespread and increasingly recognised global ecological threat. This meta-analysis investigates the effects of varying wavelengths and illuminance levels of ALAN on organisms. Broad-spectrum 'cool' light, enriched with blue and ultraviolet radiation, strongly disrupts circadian rhythms, melatonin production and nocturnal activity. However, contrary to common assumptions, broad-spectrum 'warm' light can be nearly as impactful as broad-spectrum 'cool' light lacking ultraviolet radiation, despite the predominant influence of short wavelengths on physiological processes. The impact of ALAN is not consistently dose-dependent, as even low light levels (< 5 lx) can cause substantial biological disruptions. Thus, effective mitigation strategies require tailored solutions to specific ecological contexts and should generally avoid nocturnal illumination unless clearly needed, as there is no single 'safe dose' and 'safe spectrum' of ALAN.