I have also done both - out of ORD actually - and I concur with the previous poster. I had no trouble sleeping when I arrived in London after the daytime flight. It should be remembered that I had to be at ORD in the vicinity of 4am so being fatigued at 6 pm Chicago time (midnight London) was not a problem. I also have the problem of finding it extremely to sleep in any type of moving vehicle so the overnight flight will leave me extremely on the following London afternoon/evening.