Harford only suggests that the projects should overlap in their time frames. We should keep undivided focus in our minutes, hours, and maybe even days, but seek variety in the weeks, months, and years. For example, you might want to list all your different projects and interests and try dedicating each week entirely to one of them, as much as possible focusing on a single thing but then switching to another one the following week (or month, or whatever timescale makes the most sense to you).
