Watching through TOS again I've noticed that most episodes the bridge is lit, but some episodes it's dark, even when not under alert conditions. I just realized today that whatever the technical reasons for it, in-universe it's indicating day shift/night shift like on a seafaring ship.
They did it purposefully on TNG, and then Voyager/First Contact the lights dropped during alert conditions. Voyager also did the night shift lighting a couple of times. DS9 has such a huge room for Ops that it was always lit unless the plot called for a power outage.
Add: I just remembered, The Defiant doesn't drop the lights during red alert, but it does when the ship is cloaked. Interesting distinction.
You: watches Contagion
“Wow, they built a second Galaxy-class bridge just for this show!”