Thermistor

Thermistors are semiconductor devices whose electrical resistance changes predictably with temperature. Negative Temperature Coefficient (NTC) thermistors, most common in wearables, decrease resistance as temperature increases following the Steinhart-Hart equation. The sensor is placed in contact with skin, typically on the wrist, finger, or in-ear, and resistance is measured via a voltage divider or bridge circuit. Wearable thermistors achieve resolution of 0.01-0.1°C and accuracy of ±0.1-0.5°C after calibration. Skin temperature differs from core body temperature by 3-4°C and varies with ambient conditions, blood flow, and measurement location. Algorithms account for ambient temperature and establish personalized baselines to detect meaningful deviations. Elevated nocturnal skin temperature correlates with illness onset (fever detection), while cyclical temperature patterns track menstrual cycles (basal body temperature rises ~0.3°C post-ovulation). Continuous temperature monitoring also informs sleep stage classification, stress response detection, and metabolic rate estimation.