Iam doing an health project and i would like to rephrase the last section “a lot of people underestimate getting enough sleep, but one they hear the risks, it will open there eyes from what is going on around them.”

Well, you could rephrase it in a couple of ways. 

1. Add new words.
2. Use new phrases. 
3. Change the order of the sentence. 

"A lot of people underestimate getting enough sleep, but one they hear the risks, it will open there eyes from what is going on around them.”

Ex,  A great deal of people underestimate the importance of getting enough sleep, but once they hear the risks, it will open their eyes about whats really going on around them.