The amount of hours you sleep
Fun fact: according to The Guardian, less sleep means a shorter life. Really.
But seriously, sleep is the time our body needs to build muscle, repair tissue, and solidify memories, according the National Sleep Foundation. If you're constantly sleep deprived, you increase your risk of diabetes and cancer. You are also more likely to get sick, get into an accident (ever tried driving to work in the morning after partying until sunrise?), and your sex drive diminishes.
Now, get in bed.
Your mental health
While there's no real number to attach to something like anxiety or depression, your mental health is just as important as your physical. Whether your preferred method of keeping your mind in check is through therapy, medication, exercise, self-care, or spending time with friends and family, making sure that you're OK (or working toward being OK) is far more important than a random number on a BMI chart.
Bye bye, BMI.
Can't say we'll miss you.