Why do submarines switch to ominous red lighting during night operations when they are deep underwater?

Red light protects a crew’s dark-adapted vision

A submarine’s red-lit control room is not meant to illuminate the ocean outside. At depth, there may be little or no useful daylight to see through the hull. The lighting is mainly about what happens when crew members need to look from illuminated instruments toward a much darker view—especially through optical equipment such as a periscope.

Human vision shifts in low light. The eye’s rods, cells that help detect shapes in darkness, are especially sensitive to blue-green light and much less sensitive to deep red. Keeping the control room dim and red can therefore let crew read essential displays without disrupting their dark adaptation as much as ordinary bright white light would.

The advantage depends on keeping it dim

Red light is not a magic night-vision switch. A bright red lamp can still impair dark-adapted vision, and adjusting from a brightly lit space to darkness takes time. Submarine lighting is kept subdued so crew can work while limiting that adjustment; when brighter light is needed, the tradeoff is unavoidable.

The practice reflects a practical constraint of watchkeeping: people may need to monitor instruments and then quickly make use of a much darker optical view. The red glow helps manage that transition—it does not make the deep ocean visible.