This is a very nice study. I think that you are probably correct in your conclusion that the lower melatonin is a consequence of beta receptor down regulation. It is a pity that there is no way to test the hypothesis in humans. In animals you would test the response to isoproterenol, but George Vaughan showed many years ago that the doses required in humans would be dangerous if not fatal! Melatonin would then be very low I guess!