If there is no external lighting during the hours of darkness, yours eyes take time to adjust to different light levels. When moving from one light level to near darkness, i.e. exiting a building in an emergency in low light levels, the human eye takes some time to adjust to the new lower light level.

Without the external lights people would have difficulty finding their way.

Previous editions of BS 5266-1 allowed the use of street lighting to illuminate the external areas. With local councils turning off street lights at certain times to save money and energy.

It’s not always possible to rely on the street lighting. The current edition of BS 5266-1 says that if street lighting was relied on it needs to be reassessed to make sure that they will still be illuminated at all times the premises are in use.