What are the primary proxy measurements used by consumer sleep trackers?

Motion (actigraphy) and heart rate variability.

Since consumer devices cannot easily measure direct brain waves like PSG, they rely on proxy signals that correlate strongly with the body's state of rest or activity. The primary input is motion detection, known as actigraphy, which uses accelerometers to gauge restlessness and overall movement patterns. This is supplemented by monitoring heart rate variability, which reflects the state of the autonomic nervous system. The true invention of the consumer tracker is attributed to the success of the algorithms that reliably translate these simpler movement and pulse signals into functional estimates of sleep stages, a process that abstracts the complexity of clinical measurement.