An apparatus for determining the hydration status of a user, comprising: (a) a ring, having an internal surface with an effective internal diameter, configured to be worn around a finger of the user; (b) an optical sensor system comprising (i) one or more optical emitters (1905, 1906) mounted with the ring such that light emitted by the one or more emitters is directed toward the finger and (ii) one or more detectors (1907, 1908) mounted with the ring such that the one or more detectors (1907, 1908) produce a detector signal representative of light reaching the detectors (1907, 1908) from one or more emitters after the light has interacted with tissue of the finger, configured to detect physiological signals indicative of opening and closing of the user's aortic valve; (c) a trigger system, configured to detect an event indicating a hydration measurement is to be initiated; (d) an optical sampling control system responsive to the trigger system configured to operate the one or more emitters and the one or more detectors (1907, 1908) at a first set of operational parameters; (e) an analysis system responsive to the detector signal; (f) a hydration determination system configured to determine the hydration status of the user; (g) a feedback system configured to provide feedback, further comprising a user input system configured to receive input from the user, and characterized in that the hydration determination system is configured to determine the hydration status of the user from the interbeat time interval and the ejection time interval and from the input, and by the analysis system responsive to the detector signal being configured to determine an interbeat time interval between successive openings of the user's aortic valve, and an ejection time interval between successive opening and closing of the user's aortic valve; the hydration determination system being configured to determine the hydration status of the user from the interbeat time interval and the ejection time interval; and further comprising a posture determination system configured to determine the user's posture responsive to the optical sensor system, the user input system, an accelerometer mounted with the ring, a gyroscope mounted with the ring, or a combination thereof, and wherein the hydration determination system is configured to determine the hydration status from the interbeat time interval, the ejection time interval, and the user's posture at the time the detector signal is produced. (a) a ring, having an internal surface with an effective internal diameter, configured to be worn around a finger of the user; (b) an optical sensor system comprising (i) one or more optical emitters (1905, 1906) mounted with the ring such that light emitted by the one or more emitters is directed toward the finger and (ii) one or more detectors (1907, 1908) mounted with the ring such that the one or more detectors (1907, 1908) produce a detector signal representative of light reaching the detectors (1907, 1908) from one or more emitters after the light has interacted with tissue of the finger, configured to detect physiological signals indicative of opening and closing of the user's aortic valve; (c) a trigger system, configured to detect an event indicating a hydration measurement is to be initiated; (d) an optical sampling control system responsive to the trigger system configured to operate the one or more emitters and the one or more detectors (1907, 1908) at a first set of operational parameters; (e) an analysis system responsive to the detector signal; (f) a hydration determination system configured to determine the hydration status of the user; (g) a feedback system configured to provide feedback, further comprising a user input system configured to receive input from the user, and characterized in that the hydration determination system is configured to determine the hydration status of the user from the interbeat time interval and the ejection time interval and from the input, and by the analysis system responsive to the detector signal being configured to determine an interbeat time interval between successive openings of the user's aortic valve, and an ejection time interval between successive opening and closing of the user's aortic valve; the hydration determination system being configured to determine the hydration status of the user from the interbeat time interval and the ejection time interval; and further comprising a posture determination system configured to determine the user's posture responsive to the optical sensor system, the user input system, an accelerometer mounted with the ring, a gyroscope mounted with the ring, or a combination thereof, and wherein the hydration determination system is configured to determine the hydration status from the interbeat time interval, the ejection time interval, and the user's posture at the time the detector signal is produced.