Introduction

Obtrusive light, or light pollution, is defined as ‘Spill light which, because of the quantitative, directional or spectral attributes in a given context, gives rise to annoyance, discomfort, distraction or a reduction in the ability to see essential information'. It affects people in three main ways:

• upward light pollution, known as ‘sky glow'

• light trespass

• disorientation from glare and clutter.

Sky glow occurs when light is shed upwards, either directly by light sources such as street lamps or floodlights or when reflected from the ground or other surfaces. Dust or water vapour in the air scatter the light to give an orange glow above our towns and cities, which obscures stars in the night sky. The orange colour is derived from the sodium lamps that make up most street lighting.

The UK experiences some of the worst light-polluted skies in Europe. More than half of the population is already unable to see the Milky Way in the night sky, and the problem is spreading into rural areas as artificial lighting at night increases.

Light trespass is a more localised issue, which occurs where artificial light sources are visible beyond the areas they are supposed to light. Often associated with poorly aimed floodlights or overlighting of areas, light trespass can disturb and annoy people, who may find their bedrooms lit to the extent that they cannot sleep.

Glare occurs when a source of artificial light is so much brighter than the area around it that it causes discomfort and inability to see – something that is very dangerous to road users. A common example is the effect of main beam headlights on an oncoming car when driving along a dark road, but it can also be caused by fixed lighting installations, such as a floodlight that directly faces an observer. Visual clutter occurs where important lights such as traffic signals are viewed against a competing background that reduces their visual impact. The effect is worse if the competing lights are coloured.

Whether obtrusive or not, artificial lighting can have ecological effects on wildlife in the locality, and changes the character of the rural landscape. Many of these effects are still poorly understood.

It is known that artificial light can extend the length of the day, affecting insect populations, nocturnal mammal species and nesting and roosting birds. Night-flying insects such as moths can either cease flying and settle when exposed to high levels of general illumination or fly in spirals where they are misled by an individual light. Some entomologists believe that increased street lighting is a significant factor in the reduction of moth species in urban areas.

Bright light is likely to deter nocturnal mammals from using established foraging sites. If the population is already threatened this represents an additional risk factor, particularly if the lighting is along important areas like river corridors. Conversely, if road lighting deters animals from crossing the carriageways it may have a positive effect. Fast-flying bat species can benefit from the insects attracted to street lights, but slower-flying species, which include most of those considered particularly vulnerable in Europe, do not experience the same benefits.

The behaviour of birds is strongly affected by light. Artificial increases in day length can induce hormonal, physiological and behavioural changes in birds, initiating breeding. Very bright lights can also attract and disorient birds. Nocturnal species are particularly likely to be disturbed.

Some short-day plants will not flower if the night is shorter than the critical length. Others will flower prematurely.