How bout setting an alarm maybe an hour earlier? It depends on whether you’ll be able to get up in the morning. It’s always better to get sleep since you said it’s going to be a busy day. I’d rather want you to at least have a little sleep then be groggy and tired while doing other stuff on top of that.
For the store it’s better to check rather than skip it. It’s a gamble but there’s a low chance there might not be anything you’re looking for because stores usually stock up before they open. Unless you have something else to do during that time I’d advise you to check it then just skipping completely. Though this is what I would personally do if I was in your situation. Please don’t feel pressured to do what I said because it is your choice. I hope your day goes smoothly tomorrow.

it's 3:12 a.m here in argentina, I have class tomorrow at 10:00. The thing is I have to buy a few ingredients from a store that opens at 9:00 so I'd decided to wake up at 7:30 to get ready, buy the stuff and take the bus around 9:20 to be on time... my problem is I DON'T KNOW whether to sleep or not cause I usually tend to function worse with less than 7hs of sleep than with no sleep at all... but tomorrow is gonna be a busy day and I'm scared I might be so tired I won't be able to function greatly as well so I'm here overthinking and reviewing my options
Also another possibility is the store doesn't have the ingredients I need .. i'd be screwed AND tired