i used to love the TPA-LAX flight that left at 7 am and gets to LAX at 930 AM. a few coffees/redbulls and you power through the day.. the LAX to TPA red eye though..i've done it a few times where i land at 630 AM and go straight to work. by 12 i'm a mess. i just wish delta would offer proper pillows in FC, then maybe i can sleep. However, i'm confident if i had a lie flat i would be fine.
so to answer your question; if i'm given a lie flat bed and a flight time of 5+ hours, i have no problem with it