Do exactly what you need to do in order to sleep well the night before. For some people this means pushing everything to the side and going for a nice long run. For others, it means ice cream, video games, and other brands of peaceful vegetation. For others still, the knowledge that you're not utilizing potential study time can be anxiety-inducing. I, for one, see absolutely no problem with studying or even doing practice FL's the day before. Studying can be exhausting, yes, but unless you're eighty years old, you're not going to be so spent the next day as to nullify the benefit of shoring up weak areas or having those hard-to-remember formulae fresh in your mind.
I took my last FL two days before my real MCAT and scanned my review books while watching TV on the day immediately prior.