a normal heart rate is considered as 60-100, but of course the fitter you are, the less your heart rate needs to be. 40 is really considered as bradycardia but if you are super fit, working out 6-7 days a week then this would be expected. You say that your heart rate is low when you wake up? and I think in many people this is quite normal because you are so relaxed. Our muscles are hardly used in sleep so the heart has an easier time, plus you are laying flat. My heart rate used to drop to 25-28 during sleep but that was due to medication, and Doctors seemed ok with it when they realised it was fine at that rate, not causing any harm. So long as you don't feel any dizziness, chest discomforts, shortness of breath or fainting spells, then it's usually fine. However, I do strongly recommend that you have a chat with your Doctor who MAY want to ensure your heart rhythm is in order and that you have no murmurs. How high does it get once out of bed and walking around?
Do not worry, but 40 is low. I was told until is not under 40 is OK .
But you are only 18, and have a healthy heart, so ...Tell your doctor about it. He might want to see you and you should see him.