Environment circuits

The TARDIS environment circuits, or environment systems, were the controls responsible for simulation of light within the ship. (PROSE: Falls the Shadow) The systems generated a soothing hum. (PROSE: The Well-Mannered War)

In the Doctor's TARDIS at "night," the circuits would dim the lights to darken the ship, and when the travellers woke up the TARDIS would respond by bringing the light up. Ace also wondered if the TARDIS would keep a room dark if she subconsciously wanted it, without knowing she did. When the Grey Man took control of the TARDIS, the ship became completely dark, requiring the Seventh Doctor, Benny, and Ace to use torches. (PROSE: Falls the Shadow)