Don’t cram the night before. Sleep as much as you possibly can. Eat a real, but not heavy breakfast, with only as much caffeine as you normally have. These are your goals, at least.
On the question of clothes, the advice you are most likely to get is to be “comfortable.” In blogs and forums, I have seen shorts widely recommended for warm days, sweats for cold ones. These kinds of suggestions employ a rather narrow definition of “comfort,” which does not have to mean “slacker attire,” just as “comfort food” does not have to mean “Cup Noodles.”
You actually do need layers. There is about a 50% chance the room will be freezing, especially if the test is in a warmer month. You should have a light base, a t-shirt or an oxford/blouse, and at least a light sweater or jacket ready to wear, if not a jacket jacket. Layers means no shorts/ no short skirts. You can’t layer coverage on your legs. It is safe to assume the room will not be so scorching that wearing pants will prove distractingly difficult, but if you take your seat at a cold desk in a cold room with only shorts and a t-shirt, your focus will be compromised before you start filling in your name.
The test is not a dinner party; there is no reason you cannot take it in a coat. If the room is frigid, not only will you be as warm as you need to be, you’ll be more comfortable than your peers who either didn’t come prepared or have some stigma about wearing outerwear indoors. Comfort can be competitive. If you feel like nothing can touch you when you’re in your down jacket or giant hoodie, that’s a feeling you can harness for the exam.
This is the point where my advice bifurcates a bit. Wear sweats or temperature regulating gear ONLY if that’s part of your normal look. Comfort on test day is the same as comfort during a normal school day or workday. In other words, if it would normally feel weird for you to dress like you plan on a day of Netflix, or like you are about to hike the Appalachian Trail, then don’t do that! Comfort means feeling confident, which means wearing your “best” clothes more than the most ergonomic ones. If it helps you get in the zone, dress up a bit, generally going for “business casual.”
For my own part, I like to wear a proper oxford shirt even when working at home. Real confidence comes from taking yourself seriously, and feeling put together can help establish a capable mindset. There is no single strategy for every test-taker. Be comfortable, yes, but remember that the test site is a competitive, formalized space. Picture someone you would assume did well and embody that!