Please note I'm not writing this in any attempt to throw around wet blankets. I have a background in hardware design, but I have seen many former start-up employments of my own gradually evaporate away due to sad realities of attempting to design and sell custom hardware, at least on this hemisphere. (That is, I would share your interest in hardware design, but presently am unable to pursue such myself.)
I would strongly encourage folks interested in this do thorough research about whether any existing products (e.g. Fitbit) may be adaptable to the purpose described. Designing your own hardware from the ground up and then attempting to sell it successfully will fail, statistically speaking. Especially so if there happens to be an existing product on the market that could achieve 80% of what you're wanting to do. For example, does the proposed product need to be fully stand-alone, or could it be a bluetooth peripheral for some host devices like a Fitbit or a smartphone?
I should note that none of this doom-and-gloom should apply to tinkering and hobbyist projects. It's that just economic realities of pursuing hardware design through to mass production are quite stark.