This is one of my pet hates. I do think it is better than it used to be but I remember a number of flights where the crew have told me that they specifically turn the temperature up on the "sleeping" section of the flight. This always seem unbelievably odd to me as almost everyone I know goes to bed and their heating turns off around that time! I only know one couple who leave the heating on all night! Due to this we have only stayed there once and even with it being January and us opening both windows full whack we both roasted all night!
Even in economy there are blankets to go around and even with a blanket 20+ degrees is way too hot as you are still in your clothes!!! Crazy.