Since the facility was underwater and there was no natural light, the facility was programmed to go into day and night mode to alert people on the base what time of day it was.
The base was visited by the Twelfth Doctor and Clara Oswald who arrived by orders from the TARDIS, when the base was haunted by ghosts. The ghosts couldn't manifest during the day shift but could hack into the Drum's controls and switch the shift. The crew used the Faraday cage to hide and later trap the ghosts as the ghosts couldn't get through the cage doors. (TV: Under the Lake) The ghosts were released from and later retrapped in the Faraday cage, and UNIT were contacted to remove the cage and eject it into space, where the ghosts would dissipate. (TV: Before the Flood)