I've had the lie flat business class once, as a Delta free upgrade LAX - Tokyo in 2016. I slept as soundly as in a bed for 5 hours, deliberately not trying to adjust into the new time zone because I planned to visit the 5AM Tokyo fish market Liz had visited 4 days earlier. But my day was a Wednesday, the day of the week it was closed.

I was in Tokyo about 12 hours before the ensuing flights into Indonesia.
A year ago we paid Lufthansa $200 each for lie flat from Frankfurt to Johannesburg. But that was not business class; we each got an empty row of 4 seats in the back of coach. Liz said it helped her a lot; I didn't think it was that comfortable, only slept 3 hours. It was easier for me to sleep in the Emirates A380 coach seats this March, and Liz slept well on that one too. In standard coach seats Liz rarely sleeps much at all; I sleep erratically, some flights much better than others.