PPS: Hello from Amsterdam!

My guess would be that it's so your eyes are already adjusted to the darkness, so in the case of an emergency where you needed to get off of the plane in a hurry, you'd be able to see better, and get away from the plane/to safety quicker.

That's my serious answer. i'm sure i'll come up with a silly one soon enough

Hello,

As far as I am aware Rianna's theory is correct.

The most dangerous part of any flight is intermediate to short final to land. This is about 4NM to touchdown.

It takes about 30mins for the body to produce enough of the chemical needed for good night vision. Thus lights are dimmed early so that if you have to leave the aircraft you have a greater chance of being able to see better.

Out of interest; Internal blinds are left open so that you can see which side of the aircraft to get out of and also if you are over land or water. Any emergency vehicles on the ground will also need to see in. Being able to see outside also helps with any sickness you may feel from acceleration and deceleration.

If you were to have an emergency outside of take off and landing your eyes would probably not be adjusted in time. However, if you were at cruise altitude it could take 20/30 mins to reach the ground and during this time emergency lights would be on giving time for your eyes to adjust.

EU law states that you only have to have lights out for take-off and landing.

Hope that helps.

Oh, on take off it could also be so that you can get started on joining the "Mile High Club"

The thing about motion sickness and seeing outside occurred to me too, but I didnt mention it

In answer to the question about "Why only on take-off and landing?" I'd say good night vision is probably the least of your worries if you need to leave a plane in mid-flight, especially since the only emergency equipment available seems to be life vests and a raft... maybe you'd rather not see well? :O