First, the greater your needs for the day, the less likely you’ll get them done. Scientific studies show we tend to underestimate how long something will take, often by a magnitude of two. Your quick errand will likely take twice as long as you think, which can easily throw your entire day off.
Second, the more stuff on your list, the less motivated you feel at the inset and the outset. Plowing through the to-dos may initially feel gratifying, but once you hit a speed bump (see the “quick errand” example above), you’re more likely to be frustrated by the sheer number of tasks ahead of you.
Even if you’re able to jump from task to task, it’s more difficult to maintain that momentum through several items. You may end up losing steam. And if your most important tasks are later, you could paint yourself into a corner.
Lastly, accomplishing a little is more encouraging than going for a lot. Your mind becomes more efficient and strategic when an entire task is complete, which is why multitasking is one of the least efficient things you can do. Complete a few items well, however, and you are more likely to have the clarity to do the rest of your tasks even better.