Snoozfit's Sleep Sensor relies on ballistocardiography; detecting the heart beat, breathing and other body movements from under the mattress. Ballistocardiography is a technique for producing a graphical representation of repetitive motions of the human body arising from the sudden ejection of blood into the great vessels with each heart beat.

“Ballistocardiography – Technique of graphic representation of the movements of the body imparted by the ballistic forces (recoil and impact) associated with cardiac contraction and ejection of blood and with the deceleration of blood flow through the large blood vessels. These movements, quantitatively every minute, are translated by a pickup device (transducer) into an electrical potential which is suitably amplified and recorded on a conventional electrocardiograph or other recording machine.” 

Explanation by courtesy of National Library of Medicine (NLM).