Compare: my super-smart friend is studying for a test. I know he'll ace it no matter what. I wouldn't tell him "if you go to bed now and get some sleep you'll ace it tomorrow", and I wouldn't tell him "if you study all night you'll ace it", despite both of those being true.

But this is because the purpose of saying the above isn't merely to inform your friend of a true statement — it's to convince him to get a good night's sleep, in order to cause him to be well and happy.