If you are asleep, how do you know your HR dips below 40? That's just me being curious! I have always understood that young, athletic people often have lower heart rates, which is apparently very normal, but you didn't say what your "normal" resting heart rate is. You've said that you struggled with an eating disorder in the past, but have overcome that and your weight is now "normal," yet your doctor is telling you not to exercise as you aren't eating "enough!" Since your EKG was normal, we can rule out your past eating disorder having caused any cardiac issue. So, I'm probably as confused as you are……especially since you have no symptoms. You ask if your "issues" could still be due to "under-eating," and that gave me a bit of a pause. Do you mean under-eating in the past or are you STILL under-eating even though you say your weight is normal? I can't help but feel there is a piece missing from this puzzle. Since you have overcome your eating disorder, your weight is "normal," and your EKG was good……(Not knowing your height, weight or build, I don't know if a BMI of 24 is good or not).I don't understand why your doctor says no exercise….but if it has something to do with an ABNORMALY low heart rate, it seems they should have referred you to a cardiologist. That your doctor just said "don't exercise" without suggesting other tests leaves me a bit uncomfortable. I would definitely get a second opinion and if your insurance covers it, I'd try to see a specialist in sport medicine….but if that is out of reach, a second opinion, in my humble and non-medical opinion, is called for. I wish you the best.