Originally had an 8am time slot. Anxiety combined with unsuccessful attempts to set myself on an early morning wake cycle = didn't get any actual sleep the night before. Anyway, I showed up at prometric, but they were having problems so we were sitting there for 2 or 3 hours waiting before a supervisor came out and told us we would have to reschedule.
The only available next time slot was 2pm-10pm the next week. I was usually waking up at 11am-ish and going to sleep at 3 or 4am during the study period, so it worked for me. On the night before I went to sleep at 2am after watching a movie, slept pretty well until 11am, got ready, had a nice lunch, took the test, wasn't really fatigued too much afterwards.
From a purely performance perspective, the later time slot was certainly better for me because that was on my normal sleep schedule. However, I had quite perfectly scheduled my studying strategy for that first date and having to take it 9 days later led to forgetting little points and probably negated the performance advantage of the later time slot, but that is pretty specific to my situation.
cliff notes: if you are a vampire, sleep in and do the later time slot