This question comes up periodically... here is a thread I found in a quick search:
https://www.flyertalk.com/forum/amer...questions.html

Essentially most of what I remember seeing seems to focus on the cabin temperature.
I've only flown JL in a premium cabin once; it was an op-up SGN-NRT, so I really can't complain too much, but it was rather warm. Other than that (it was a daytime flight), things were good.