Eating poorly; sleeping poorly; having too much dust, filth, or random junk around the house; not going outside; not exercising enough; not getting social interacting in the minimal needed amount, obsessing over the wrong aspects of your work.... This sort of things.
Not all of these are problematic for all people, but to the extent some are problematic for a given person, they tend to form a vicious cycle - people do bad things to themselves from lack of awareness or temporarily depleted willpower, and those behaviors in turn cause even less self-awareness and reduce the willpower reserves further.
For that reason I am contemplating some sort of a software-based aid that helps one person to periodically check on the other person's daily routine from a distance to make sure they haven't fallen into this sort of a "depression trap". I tentatively call it "a brother's keeper" project, although realistically it's more likely to be the subject's parents or childhood friends who will bother to check daily on how many minutes he or she spent outside and how many meals did they have per day, and whether there were enough nutrients in it. The point would be not to nag the subject daily, but to catch the moment they are falling off the bus, so to speak, and either intervene, summon other friends, or get professional help.