Estimation

[LetYouDown]

Not really a probability question, but I figure this is the most appropriate forum.

How many feet above/below sea level is the average human being at any given time, on average? Does this fluctuate dramatically over any significant period of time?

Obviously a legitimate answer is all but impossible to determine accurately. More concerned with the thought process you'd use to approach this problem and the margin of error you can narrow it to.

1.) People considered must obviously be alive.
2.) Assume we're looking at the lowest point of the individual. Someone asleep would be where they touch the bed, someone standing up with be their feet, someone upside down would be the top of their head, etc. etc.