A device for measuring data indicative of a physiological activity of a user and being wearable on a body part of the user, the device comprising: a band configurable to be fitted over the body part, and having an interior surface contacting the body part when worn by the user; modular sensing circuitry removably disposed on the interior surface, being configurable to be in contact with the body part; and a plurality of light sources disposed on the interior surface adjacent the modular sensing circuitry, and configured to emit lights onto the body part, the modular sensing circuitry configured to receive data indicative of a photoplethysmogram, PPG, signal of the user; wherein the modular sensing circuitry further includes a sensor and a sensor computing unit coupled to the sensor, and wherein the sensor computing unit further comprises a galvanic skin response analogue front end, GSR AFE, an optical sensor analogue front end, a processor, a memory, a three-axis accelerometer, a pressure sensor and a battery, the device characterized in that the device further comprises a base module, wherein the base module further includes a base computing unit that includes a processor, a memory, a communication interface and a set of sensors including an accelerometer and a thermometer, wherein the band is removably coupled to the base module such that the band is replaceable with a different type of band; and the sensor computing unit further comprises an ECG and bioimpedance, BIOZ, analog front end, AFE, and an analog-to-digital converter, ADC; and the processor is configured to execute a calibration and data acquisition component for performing sensor calibration and data acquisition functions, wherein sensor calibration includes a process for self-aligning one or more sensor arrays to a blood vessel. a band configurable to be fitted over the body part, and having an interior surface contacting the body part when worn by the user; modular sensing circuitry removably disposed on the interior surface, being configurable to be in contact with the body part; and a plurality of light sources disposed on the interior surface adjacent the modular sensing circuitry, and configured to emit lights onto the body part, the modular sensing circuitry configured to receive data indicative of a photoplethysmogram, PPG, signal of the user; wherein the modular sensing circuitry further includes a sensor and a sensor computing unit coupled to the sensor, and wherein the sensor computing unit further comprises a galvanic skin response analogue front end, GSR AFE, an optical sensor analogue front end, a processor, a memory, a three-axis accelerometer, a pressure sensor and a battery, the device characterized in that the device further comprises a base module, wherein the base module further includes a base computing unit that includes a processor, a memory, a communication interface and a set of sensors including an accelerometer and a thermometer, wherein the band is removably coupled to the base module such that the band is replaceable with a different type of band; and the sensor computing unit further comprises an ECG and bioimpedance, BIOZ, analog front end, AFE, and an analog-to-digital converter, ADC; and the processor is configured to execute a calibration and data acquisition component for performing sensor calibration and data acquisition functions, wherein sensor calibration includes a process for self-aligning one or more sensor arrays to a blood vessel.