Human Body Communication (HBC) has recently emerged as an alternative to wireless radio wave communication for connecting resource constrained devices around the body. HBC utilizes the human body as the communication medium between the devices and provides higher energy efficiency and enhanced security enabling applications such as remote health monitoring, biometric authentication etc. As these applications require data transfer in small bursts, it is not necessary to keep the higher power consuming main receiver awake all the time. This paper carries out architectural and circuit level analysis considering HBC channel characteristics to find the optimum wake-up receiver architecture. This leads to the proposal of a narrowband HBC wake-up receiver which operates at sub-nW power. Design trade-offs for choosing the architecture (narrowband vs broadband) and system level parameters (frequency of operation, modulation technique) has been discussed by utilizing the appropriate human body channel models. Simulation results using TSMC 65nm technology shows 694 pW operation even under channel loss conditions as high as 80dB, potentially enabling applications with energy harvesting.