Health monitoring and guidance

A wrist worn device configured to determine whether a user is in a stressed state, the wrist worn device comprising:

• a photoplethysmographic (PPG) sensor configured to generate a PPG signal;
• at least one programmable processor and a non-transitory, machine-readable medium storing instructions which, when executed by the at least one programmable processor, cause the at least one programmable processor to perform operations comprising:
• receiving, at the at least one programmable processor, the PPG signal communicated by the PPG sensor;
• detecting a plurality of heartbeats of the user based on the PPG-signal;
• determining a heart rate of the user based on at least the plurality of heartbeats;
• determining a heart rate variability (HRV) of the user based on the plurality of heartbeats;
• determining a respiration rate of the user based on a low frequency component of the PPG signal;
• and
• determining whether the user is in a stressed state based on the heart rate, the HRV, and the respiration rate, wherein determining whether the user is in a stressed state based on the heart rate, the HRV, and the respiration rate comprises determining a mathematical model for the user, causing the heart rate, the HRV, and the respiration rate to be used as inputs into the mathematical model, and causing the mathematical model to output the determination of whether the user is in a stressed state based on the inputs, wherein the mathematical model comprises a weighted combination of the heart rate, the respiration rate, and the HRV for the user, wherein the heart rate is weighted more heavily than the respiration rate or the HRV;
• and
• a user interface controlled by the at least one programmable processor, the user interface controlled by the at least one programmable processor to display information related to the determination of whether the user is in a stressed state.