A patient support includes a mattress with an inflatable bladder, where the inflatable bladder has a cavity and an upper bladder wall forming an outwardly facing side for facing and supporting the patient on the patient mattress and forming an inwardly facing side facing into the cavity. The patient support further includes a light transmitter operable to emit light through the cavity to impinge on the inwardly facing side of the upper bladder wall and a light receiver operable to detect a reflection of the light off the inwardly facing side of the bladder wall, with the light receiver generating a signal responsive to detecting the reflection. A controller is in communication with the light transmitter and is operable to determine immersion of a patient into the mattress based on the signal from the light receiver.