An electromagnetic wave sensor for determining a hydration status of a body tissue in vivo includes an electromagnetic wave transmitter, a waveguide, and an electromagnetic wave receiver. The electromagnetic wave transmitter is configured to emit an electromagnetic wave signal in a frequency range between 1 Hz and 1 THz. The waveguide is coupled to the electromagnetic wave transmitter. The waveguide is adapted to be arranged next to the body tissue such that a fringe field of the electromagnetic wave signal guided by the waveguide penetrates the body tissue. The electromagnetic wave receiver is coupled to the waveguide. The electromagnetic wave receiver is configured to receive the electromagnetic wave signal modified by the body tissue in dependence of a hydration status of the body tissue.
