The dimming of cabin lights only happens when it is dusk, dawn or dark outside the aircraft. This is a safety measure, and is to ensure your eyes are adjusted to the gloom enough to see the floor lights leading you to safety along the aisle in the event of a crash or emergency evacuation.

Dimming the cabin lights during takeoff and landing is a critical safety protocol designed to prepare passengers' eyes for a potential emergency evacuation. Because these are the two most statistically risky phases of a flight, airlines want to ensure that if a power failure occurs or if smoke fills the cabin, your eyes are already "pre-adjusted" to the lower light levels outside. If the cabin were brightly lit and you suddenly had to exit into the dark night, you would experience "temporary blindness" as your pupils struggled to dilate, which could cost precious seconds in a life-or-death situation. Furthermore, a dimmed cabin makes the illuminated "emergency exit" floor path lighting and signs significantly easier to see. It also reduces glare on the windows, allowing flight attendants and passengers to see clearly outside—an essential task for identifying hazards like fire or debris before opening an emergency exit door. While it may feel like a mood-setting gesture for a late-night arrival, this practice is a science-backed requirement by aviation regulators worldwide to maximize survival rates during an incident.