A wearable device for estimating the blood glucose level of the wearer, the device including a source of near-infrared radiation arranged to irradiate a measurement area of the skin of the wearer； a spectrometer arranged to receive reflected or transmitted near infrared radiation and output an absorption spectrum comprising the absorption intensity at a plurality of wavelengths, wherein all of the wavelengths are within a portion of the near infrared spectrum having a substantially constant optical penetration depth； a processing unit configured to: receive the absorption spectrum from the spectrometer； calculate an estimate of the blood glucose level of the wearer using a stored calibration model describing the relationship between near infrared absorption and blood glucose level.
