Studying hard is obviously important, but you will see the biggest immediate difference once you go from sleeping less than 6 hours to 9 or 10 hours a day. Over the summer, I've been doing STEP modules. I got through them without a single problem while watching suits. I slept badly ONE day, and I'm struggling to even get through the prep questions, and this is an easier module than anything I did yesterday. Same thing happened in my GCSEs. Kept doing horrendously (for my standards, which was 9999876651 pred) and ended up with 9999998888 which I was honestly unlucky to get (1 mark off in 2 and 3 off in another) and all my 9s were pretty far above (I think my worst was eng lang which was 8% above). I went from that pred to that achieved in 1 month. You know what changed? Not studying, I started going to bed at 11:00 every day. It's not optimal, but it's FAR better than going to bed at 2AM like I used to. Do not stay up all night or late studying. Just go to sleep. It'll do you far more good. All the exams I didn't get 9s in, I stayed up late studying for and they were some of my best subjects (comp sci, which I got 94% in in mocks, chemistry which i got 90% on on 2022 past paper series and music where I got 116/120 for coursework and lit was the only actually bad subject). The exams I did well in, I went to sleep early the night before, woke up refreshed, went about my day, studied for 15 minutes before exam and did great.