There's a significant discussion going on about the effects on your brain when you do a context shift between disparate tasks in Pragmatic Thinking and Learning, something that inevitably takes place when you work on one project and then jump to working on another project.
You should read up the Human multitasking wiki, but from a perspective that your not doing multiple tasks at the same time, rather that your taxing your brain to load up several large contexts in a single work session.
Here's some excerpts taken from the Wiki:
Some believe that multitasking can result in time wasted due to human context switching and apparently causing more errors due to insufficient attention.
When people attempt to complete many tasks at one time, “or [alternate] rapidly between them, errors go way up and it takes far longer—often double the time or more—to get the jobs done than if they were done sequentially,”... This is largely because “the brain is compelled to restart and refocus”.
People have a limited ability to retain information, which worsens when the amount of information increases.
Without much background info, PT&L defined this analogy of your brain's context as fast and accessible storage for your consciousness where you could say that your current context is something like a page file. Dumping a large page file and loading a new, significantly large page file is described as a physical feat that can be performed for a limited number of times per day and that performances degrade every other time. Basically, you end up with a fragmented context, neither are you on the current project, nor the previous one.