Here's some unsolicited advice for improvements:
* Focus relentlessly on the calculator experience. Why am I pressing "calculate"? What are some ways you could rework it so that I can give quicker input and get more meaningful output? Your goal should be to reduce friction. Maybe let me type in the time in addition to having the android-style clock widget. Definitely show me a series of clocks as I'm updating the picker as I change the time. It's important to let your users play so that they can intuit about the concepts. Also, maybe show extra times for oversleep/undersleep, but accented in such a way that the user knows they're not healthy choices. Maybe include "in x minutes" with the times too. Maybe start the page with the current time + 8 hours selected, showing the live updating output.
* Bury all the sleep health and science stuff. It's probably great for SEO, but the star of the show here is the calculator widget. If anything, maybe instead have a short promotional video that explains the science/health stuff briefly and explains what the calculator does.
* This would be more useful on my phone, possibly as a native widget. And it would be useful paired with an alarm clock. I'd really like an app (ideally with a widget) that shows me wakeup times based on the current time and lets me quickly set alarms. I always check my phone right before I go to sleep anyway, so it would be easy to click a widget and get proper alarms set. You're probably going to need to learn React Native at some point anyway, might as well do it now.
Take this simple calculator, polish it to absolute perfection, give it away for free, and promote it wherever you can. Having one successful app at your age, even if you don't make a dime, will open up so much opportunity for you that it's 100% worth it.
Also, it's super hard to get software "right", and it's a drifting target. Look at how much Apple and Google have changed their basic Clock apps over the years. I think doing everything perfectly is usually terrible advice, but at your age and with the simplicity of this project, it's worth trying to be a perfectionist. You'll learn that everything is a lot harder than it looks and that you should be terrified of software development :)
Side rant: OP led with "I'm 15", which makes sense, and this wouldn't have frontpaged without that context. I understand that touches a nerve with people who believe the frontpage should be 100% dictated by objective merits. If this place gets overrun with industrious 15 year olds trying to show off their projects to an audience, well, we could have worse problems than that.