I get high 180 heart rate at night but woke up this morning with a heart rate of 36. what can cause such variability?

Arrhythmia: A heart rate of 180, given your age, is approaching the maximum heart rate that doctor's expect when doing "usual" activities (in this case, exercise) without an underlying disorder. If u get this heart rate while sleeping or resting, then u could have atrial fibrilllation (A-fib) or another arrhythmia. In the morning, ur heart compensates by slowing down to rest (the only way). See a cardiologist