Every decision you make taxes your brain1: From choosing the shirt you'll wear today to browsing through your photo album, reading incoming messages, or deciding wether you'll eat chocolate or not. A portion of your daily "thinking power" goes away every time you put your brain to work. (For instance, you're more likely to do math better in the morning than right before going to bed.)
This is what researchers Sendhil Mullainathan and Eldar Shafir studied: "[B]eyond time, tasks also use [thinking power], some more, some less." The order in which you think and make decisions matters. Do what you care the most about first, and leave non-important, mindless tasks for later in the day. Otherwise, you'll be wasting your ability to make good decisions.
If you can't think anymore today, don't worry. Your "thinking power" will be back tomorrow — all you need is eight hours of sleep.
Scarcity by Sendhil Mullainathan and Eldar Shafir. ↩