The honest answer is that the night before matters much less than the three weeks before it. But it can be actively damaging, so it is worth handling deliberately.
Do not learn anything new
A technique acquired at 11pm will not be available under pressure at 10am. Worse, it displaces sleep and raises anxiety by reminding you how much you do not know. The topic you discover tonight was not going to be the difference.
Re-read your own mistakes
The highest-value hour is going back over problems you got wrong during preparation, and specifically why. Not the solutions - the failure mode. "I forgot to condition", "I misread with-replacement", "I rushed the arithmetic".
Those patterns repeat, and having them fresh is the closest thing to a real edge.
Warm up the arithmetic, briefly
Ten minutes of mental math the evening before and again in the morning. Not to improve - to arrive warm rather than cold. The first three questions of a timed screen are disproportionately expensive to fumble.
Check the logistics
Which pipeline, which format, how long, what is allowed. Pen and paper permitted? Calculator? Penalty scoring? Video or phone? These are knowable facts and getting one wrong is an unforced error.
Prepare your own questions
Almost every process ends with "do you have questions for us", and it is assessed. Have two that could not be answered by the careers page - something about how the desk actually works, or what the first year looks like in practice.
Sleep
These processes select on accurate arithmetic under time pressure, and that is the first thing to degrade when you are tired. Sleep is not the thing you sacrifice to fit in more preparation - it is part of the preparation.
On the day
Eat. Arrive early. Read each question twice before starting - misreading is a much more common failure than not knowing.