The invention is an optical sensor whose output represents or measures movement of the skin of the suprasternal notch, this movement being a measure of respiratory effort. The use of a trough detector, with a time constant long compared with a breath but short compared with the interval between changes in body position, allows detection of the optical signal corresponding to zero effort. Subtraction of the trough signal from the optical signal produces a self-zeroing effort signal which automatically compensates for changes in body position. A soft, low irritant adhesive material, which may be left in place on the skin for several days, allows the sensor to be placed and removed as often as desired without injuring the skin.