What really impresses me is the site’s collection of information and its presentation. It is very thoughtful. Not too much. Not too little. You even included sources for most of your information. Fantastic! Don’t lose focus on your writing skills as you develop your technical skills. Both will serve you well. Great work!
If I may make a few suggestions, please consider:
1. The calculator says you should try to “fall asleep at <time>.” Maybe calculate a “bedtime” by including “time to fall asleep.” Default to 20 minutes, but allow the user to adjust.
2. The first sentence tripped me up. “ You should go to bed 5-6 90 minute sleep cycles before your wake up time.”
a. It should be 90-minute. See rule 4: https://www.grammarbook.com/punctuation/hyphens.asp
b. Consider breaking it into smaller sentences that lead the reader to your point. Something like:
People sleep in 90-minute cycles. They need five or six complete cycles to feel refreshed. Waking up mid-cycle feels worse than waking up earlier at a cycle boundary. Use this calculator to determine an ideal bedtime based on when you need to wake up.
Anyhow, just some minor thoughts. Overall, it is a great site, and not just a for a 15-year-old.