And even if they were based in the east, how do you know that they were not well rested prior to departure? You are making an assumption(and y are making the suggestion) that this crew had to be tired. I suggest that you are just making guesses as potential excuses.
We're still conflating fatigue with circadian rhythm. It doesn't matter if the crew was well rested. If the pilots were flying "out of sync" with their biological cycle, then their cognitive performance might be compromised.
There's a lot of research indicating that most of us cannot perform optimally past 2am unless we
only work nights for an extended period of time -- e.g., doing night shifts exclusively for several weeks in a row without break.
Yet as soon as we switch to day work (even for just one day) our body clock "resets", and our night time alertness suffers again -- regardless of how "well rested" we might be.