Method and apparatus for monitoring the sleep state condition of an individual by using an external probe applied to a peripheral body location, such as the individual's finger or toe, for detecting changes in the peripheral vascular bed volume of the individual. A predetermined pressure field is applied to the distal end of the peripheral body location, including its distal-most extremity, to prevent the occurrence of venous pooling within and distal to the peripheral body location. The probe produces an output corresponding to changes in the peripheral arterial bed volume at the peripheral body location, which output provides an indication of the sleep state condition of the individual. Such information is useful in diagnosing and/or in treating, a number of sleep disorders as well as other conditions, such as impotence, diabetes, and various disorders in children.