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As somebody who flew quite a few lie-flat business flights on JFK->LON - you absolutely do not get good night sleep. Flight time is 7:30 and you can realistically go to sleep 30 minutes after take off (when they start serving food and other passengers are still noisy - good luck falling asleep quickly) and you are generally woken up about 1-1.5 hour before landing (Why? I have no idea but they turn on the lights, serve breakfast and do announcements about weather in London about that time before landing). So you get at most 5.5h of mediocre quality sleep which is better than nothing but not enough to function 100%.

3 hour flight will change a lot of things for JFK-LON business trips as they open up opportunities to loose only 1 full day during business trip instead of loosing two full days. I generally stay on NYC time with meetings in the afternoon LON time if trip is less than 5 days so short flight is game changer.