step 1 is about knowledge but also endurance. you need to be in the frame of mind to approach each question freshly and try to figure out what the question writer is asking.
Feeling personally refreshed and relaxed and being in a peaceful state of mind the day of the exam will help you do your best more than studying an extra day. i probably "peaked" my studying a week before i actually took step 1.. ended up moving my test date up 5 days because i was so sick of studying that I was going through first aid again and again and doing 100 uworld q's a day but not even learning anything because I was burnt out. When you get to that point, you're ready to take the test. You're not going to improve your score by studying more.
the day before every massive test in med school i would wake up early, do something active (hike, surf, run) all day long, fall asleep relaxed and ready the next day.