From baggage checks and passport stamps to the safety briefing and the resetting of tray tables and passenger seats back to their upright position, there are a number of things that happen on pretty much every plane journey—no matter how long or short, and no matter which airline or aircraft you travel with. But of all these blandly regular events and experiences, this is one of the most puzzling.

So why do planes dim their cabin lights during takeoff and right before landing?

The Passenger Safety Reason

Well, as with practically everything else to do with air travel, the regular dimming of the cabin lights of an aircraft is all part of a well-practiced and well-reasoned rigmarole designed to keep you safe.

On the one hand, if you’re a frequent flyer, you’ll know that the dimming of an aircraft’s lights is a sign that your flight has been cleared for takeoff, or else, if it is toward the end of your journey, that your plane is preparing to come in to land. Unsurprisingly, this timing is not a coincidence: takeoff and landing are well known as the more potentially perilous parts of any flight, and the change in the brightness of the cabin lights is tied to the potential risk of an emergency.

If there were to be a power outage on the aircraft, for instance, the lights would likely go out completely. If you had been happily sitting there in the full glow of the plane’s fluorescents when this happened, this sudden darkness could prove discombobulating and hamper your ability to find your way to safety. Dimming the lights at these most potentially risky parts of the flight therefore acts as a means of subtly prepping your eyes for darkness, giving them a chance to adjust to the dimness and thereby making it easier for you to navigate the aircraft in the dark (in the unlikely event that you would ever need to, of course).

The Pilot Safety Reason

On the other hand, though, the dimming of the lights isn’t entirely for the benefit of the passengers. If your flight is coming in to land at night, for instance, the glare from the interior lights of the aircraft could produce a glare on the runway—especially if the tarmac is wet. Any such glare could prove distracting to the pilot, outshine runway lights and safety lights, or disguise any potential obstacles or debris that should be avoided.

Dimming the lights on landing, ultimately, reduces the potential risks of this excess light, ensuring a safe, smooth, and problem-free end to your journey.

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