I've felt the same way and I think it is because even though the night before the race affects performance a lot, I think that the quality of sleep for the last couple of days in general affect it the most.
So in other words, if you fall asleep at 9:00 pm every day of the week except friday, where you fall asleep at 12:00, you'll run the race of a lifetime and wonder how it happened.
Or it can be the other way around. Every night of the week you fall asleep late, say 11:30, and then friday night comes along where you fall asleep at 9:00 and f up the next day and wonder how that could have happened...