The invention provides a physiological probe that comfortably attaches to the base of the patient's thumb, thereby freeing up their fingers for conventional activities in a hospital, such as reading and eating. The probe, which comprises a separate cradle module and sensor module, secures to the thumb and measures time-dependent signals corresponding to LEDs operating near 660 and 905 nm. The cradle module, which contains elements subject to wear, is preferably provided as a disposable unit.