Did you wake up at 7a every day for at least the week before the exam (ideally 2 weeks before)? If not, then yes, that would likely have had a significant impact.
It is possible to schedule afternoon appointments, although they are harder to get. If you have to take it in the morning again, you have to get yourself on that timing routine ideally for at least two weeks before the test - every day.
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I kept reading the question again and again.
Yes, it sounds like you started to lose mental stamina, which might have been due just to the morning thing but there may have been other contributing factors. Did you burn yourself out at all the few days / week before the test? Or did you take it relatively easy the last couple of days?
You say you took your practice tests under official conditions - including the essays, yes? If not, that would have contributed to a verbal stamina issue as well. Did you take the practice essays as seriously as you took the real essays? Used the same amount of mental energy to complete them?
Ah, and you also mention nerves (heart started beating faster). Did you feel like you weren't doing well? Did you know you were getting mentally fatigued and you started worrying about that?
Let me know on the above, but it sounds like we need to do something about your mental energy / stamina, nerves, etc. And, yes, you could probably use additional practice at comprehension, since that's where you then started to falter on the test.