Okay, the scenario is this: a federation ship(let’s say Voyager) is being threatened by some alien menace(let’s say the Borg). The commander(let’s say…oh, you know)then gives the order for Red Alert, then the lights dim and the red lights flash and etc, etc.
My question is this: In a situation where you would think you’d need everyone working at prime effeciency and the slightest mistake could get everyone vaporized WHY WOULD YOU DIM ALL THE LIGHTS??? :smack:
Inquiring minds want to know.
For one possible reason: It’s a visual, as well as an audible alert. With specific responses trained into the crew members for their individual duties.
Of course, the real reason that it were done in the TV may well owe more to the idea that the writers and/or the audience expected it from watching WWII naval movies. 
They do this on submarines. It makes certain readouts that are only used in battle situations easier to read.
[TV reason]It’s a visual clue that they are at alert status, and a holdover from old submarine movies.[/TV reason]
[fanwank]The lights are dimmed so that the information on the viewscreens is more visible, with less distraction from reflections. [/fanwank]
Dramatic license.
Also, it’s a signal to the crew that SOMETHING IS GOING ON. Presumably in Starfleet Academy they’re trained to function in that, & be more focused.
Also, Starfleet keyboards are illuminated, FWIW.
Could it be a visual stimulus to the crew that they are now to shift gears to battle readiness? Just a guess.
“Dim all the lights, sweet darlin’, cause tonight its all Janeway…” :eek: 
Just a caveat - I’m not a bubblehead, and have never been crazy enough to serve on a ship that sinks on purpose - but the rule for red lights inside subs, and for surface vessels, is only for those personnel who will be expected to look outside at night - so there are baffle areas around the doors going to the weather decks so lookouts can acclimate their night vision before reporting to their station. And for the command deck of a sub, even if it’s submerged, if it might be coming to periscope depth, anyone using the periscope needs to have their night vision intact so they don’t waste precious seconds reacclimating.
The engineers and other personnel who don’t need to see outside the ship never see the red lights.
It’s too let the crew members listening to their iPods know something is going on. 
Seriously, in the beginning of “Corbomite Maneuver” Kirk is getting a check up, the alert goes on, and he never sees it, facing away from the light. Dimming the lights would have let him know even when McCoy didn’t tell him. I don’t recall red alerts dimming lights in ToS, though.
Dimming the lights is a sign that they’re about to get it ON, baby!
To conserve power for weapons and defensive systems?
The lights are nothing compared to the energy required for weapons and defense. It would be analogous to shutting off a faucet in Rio to make sure the Amazon doesn’t run dry.
Only on Kirk’s ships. And maybe Riker’s, once he actually accepts a captaincy.
For the same reason that enormous amounts of energy are devoted to making the ship whoosh as it travels through space?