I would be more productive if I worked nights and weekends, but since nobody would pay the bill rate premium that inconvenience merits, it benefits nobody for me to do that.

Sometimes programmers actually want to interleave multiple projects. Maybe you have two totally different codebases to work on; perhaps one is backend async code, and the other is Javascript UI. Both are a week's worth of work, but you'd burn out if you did either straight through.

Fine: interleave the projects. But that decision is orthogonal to your billing increment. Bill full days exclusively.

You will find as your business matures that every single client you want to keep won't blink if you round that 5-hour project they need up to a full day. Call it your minimum billable increment. Offer, if you want, to chunk a bunch of small projects into a day; this is one way to prospect for more hours at a long-term client.