Wearable devices which capture and analyze physiological data in a non-invasive and not restraining manner are increasingly popular, but usually employ specialized hardware. Software is often limited, specific, and expensive. This paper shows how system-level modeling, virtual prototyping and electronic design can interact to help designing low-cost wearable bio-medical devices. We propose a design method which allows validation against actual electronic/mechanical prototypes and present a first case study, a daytime sleepiness detector.