1. A method of monitoring at least one physiological property of an organism, comprising: directing energy at a target region of the organism and at a region adjacent the target region, wherein the target region comprises substantially more blood vessels than the adjacent region; detecting an energy response signal from the target region and an energy response signal from the adjacent region, wherein the directing and detecting steps are performed by a device worn by the organism; processing the detected signals to produce an extracted energy response signal; and comparing the extracted energy response signal with a physiological model to assess a physiological condition of the organism.