13. How much sleep your body needs.
At minimum, you should be getting the recommended seven to nine hours of sleep each night, but every body is different. And when you go to sleep also matters. Do you need to get into bed at 10 p.m. to make sure you're asleep by 11? Whatever works for you, do it.
Ultimately, you know how you operate better than anyone else. Being in tune with your own body and mind? Now that's adulthood.