Not only does artificial light at night change the spatial and temporal structure and intensity of natural light cycles, it also occurs with spectra different from that of sunlight, moonlight or starlight. Some types of artificial lighting are restricted to narrow bandwidths (e.g. low-pressure sodium lighting emits a single narrow peak in the visible spectrum at 589.3 nm). Others emit over a wide range of wavelengths (high-pressure sodium lighting emits a yellow light; high-intensity discharge lamps emit a whiter light, with significant peaks in blue and ultra-violet wavelengths; and LED-based white street lighting typically emits at all wavelengths between around 400 and 700 nm). The prevailing sources tend to vary from one region to another, there is, however, a trend towards the adoption of lighting technologies with a broader spectrum of ‘white’ light.
Shifting and broadening the spectra of street lamps may lead to unforeseen environmental impacts because the spectral signature reflected from objects is an important cue that guides a number of animal behaviours. The switch away from sodium lighting to ‘whiter’ light technologies is likely to have adverse effects on the environment by increasing the spectral range over which impacts occur, and increasing skyglow in the vicinity of urban areas due to greater Rayleigh scattering at short wavelengths.