Most airlines have cabin lights turned off or on the lowest setting before takeoff and landing at night. Is this a regulation or a best practice?
Having cabin lights turned off or adjusted to outside lighting is standard practice, not regulatory. Most, if not all airlines have this as their procedure because it’s the best safety practice.
Our eyes can take 10 ‐ 15 minutes to fully adapt and see under dark conditions. To help the eyes adapt to outside darkness, after flight attendants go to their jumpseat prior to takeoff or landing, the main cabin lights are turned off or turned to a very low light setting.
As you know, passenger and crew safety is very important, and by having the lights in the cabin adjusted to the same, or darker than outside conditions, both crewmembers and passengers’ vision will be better adapted to the outside light should the need to evacuate occur.