For the past two nights, i have been having leg heaviness at rest. this doesn't occur during the day. what could be the cause?

One must consider..: ..a number of variables to answer this: your age, amount of weight-bearing activity, smoker vs nonsmoker, etc. One of the major causes of leg heaviness is venous insufficiency/vein sluggishness, but you are young for that. I would consider underactive thyroid, B12 deficiency, calcium or magnesium deficiency, Don't smoke, consider eval with your doc, exercise moderately, consider support stockings