In a simple explanation, the human eye has two types of light receptors – cones and rods. The cones are for daytime vision and the rods are for nighttime vision. During the day the rods are shut off by bright light. At night the rods are activated and have about twice the sensitivity as the cones. Also the rods have no sensitivity in the red region and do not respond to the red light – this is the reason for using red light in submarines and other naval vessels. In a room lighted only by red light, the eye responds with the cone receptors and the crew can perform their duties while their eyes becomes dark adapted. The rods do not become activated quickly. Night watches would pre-condition their eyes in quarters lighted only by red light before assuming a watch topside. I understand that sometimes the crew would also use goggles with red lenses to condition their eyes.