Insomniacs have more wakeful hours available to them, so, statistically speaking, they're likely to be overrepresented amongst active HN users at any given moment, resulting in a boost to sleep and insomnia related articles.
Sleep is one of the three pillars of health. There are also many fitness and diet articles that gain traction.
As to why health is popular on HN, it's hard to be interested in the other stuff if you're dead! (I don't actually know why, I'm interested in these all personally, but nearly never upvote the articles myself)
Well, many developers probably aren't in the best health because the job requires sitting all day. Everyone wants to be healthy so there will be a natural demand for articles about it amongst developers.
That's a good question. Maybe its because we are so hungry for information here on HN. Reading HN articles is a mild addiction for me. Perhaps not as detrimental as other addictions but the one adverse thing it does affect is my sleep.
To me, writing software is primarily a research activity. It always has been, no matter how proficient I think I am at a language. That's probably why HN articles appeal to me so strongly; it's what I do all day long.