Yes, 5 o'clock is the hour. The point at which my mind starts to close, my patience wanes, and my body begs for a comfy chair and a good SILENT book.
So I make some coffee. (Or, more likely, reheat a cup from the morning's brew. Who has time to make fresh coffee?)
That 5 o'clock coffee gives me a moment's respite. The smell of it revives me just a bit and gives me the caffeinated energy to make it over the last hurdles of my day.
Like my 5pm hump, I find there is a point of final exhaustion in almost every book. There is that last great adventure on which the protagonist must embark. There is one final villain left to be outsmarted. There is a point where everything seems fine, only for horror or terror or danger to be thrown back into the mix.
How does he or she get to the end? Most often, they pull from within. Maybe a child or a loved one is in danger if they don't solve the mystery. Maybe their own life is at stake. Perhaps world as they know it will come crashing down if they don't muster this last bit of energy and resolve.
Sometimes they pull from the world around them. Drugs or coffee to wake their senses. A weapon to protect them on their quest. Information gleaned from the Internet or some futuristic device.
Whatever the source, it wouldn't be a satisfying book if the MC just gave up. If they threw in the towel so close to the end. If they saw that final challenge and said, "Nah, not worth it." Who would read a book like that?
Do you have a 5 o'clock hurdle in your daily life? What is your "coffee?" How about your characters? Do you enjoy writing their last journey, their final success?