Have you ever noticed how tired you are after a day consumed with decision making? Picking paint colors, planning a large seminar, selecting candidates for job interviews: all these decisions place the same underlying pressure on your mental processes. In much the same way as your body cannot tell the difference between good stress (the excitement of a fabulous vacation) or bad stress (automobile accident), your mind cannot sort out significant decisions from what you might consider life's minor choices.
At the end of a day of repeatedly making decisions, you can feel barely able to choose which sofa to crash on.
There is some fascinating new research that links this feeling of fatigue to glucose depletion (see this article in The New York Times for a detailed discussion). There are many implications (dieters, take note: glucose plays a role in what you might condemn as your low will-power). The point I want to emphasize here is that the mere process of repeated decision making can exhaust you.
This becomes a greater concern when your freedom is curtailed by things you cannot control. For example, let's say it is your job to design the uniforms for a new fast food restaurant (yellow and red polyester). Let's also stipulate that your native taste is traditional and sophisticated (cashmere and pearls). For this assignment, therefore, you are working within a framework that is outside your comfort zone (it is unlikely you would ever wear the color combinations, the fabrics, or the designs you are considering). You are working from demographic information about the restaurant's clientele (this is not pegged to your own social location, so you are out of your element again). You are charged with making the decision based on criteria that don't apply to your personal choices about clothing ("must fit a large variety of body sizes and shapes;" "must be water resistant; "must hold up to commercial washing machines").
You are perfectly capable--gifted, even--when it comes to making these decisions, regardless of whether they align with your own personal taste. However, you cannot suspend your native tendencies. As a result, your cashmere taste is going to have to be put aside repeatedly in order for you to do your job. This takes energy. This is what I mean by constrained freedom in the decision-making process. (Those in poverty experience this constraint in nearly everything they do, for example, because they don't have the financial resources to exercise their own taste or judgment.)
Back to your job: you spend the whole day looking at fabric samples, talking to vendors, reading statistics -- all in the service of meeting your goal of selecting a new uniform. Now you go to the supermarket. You are appalled to realize that, once again, you can't even begin to think clearly about what to fix for dinner. I'll bet you can't even decide between plain and onion bagels by this point. Not only that, odds are that The Bagel Decision feels like a bigger deal than managing all the details you've sorted out all day long.
No matter what kind of decisions you're making, from designing restaurant uniforms to ticking off boxes on a multiple choice test to determining who will sit where at the reception, you will do a much better job and feel much better at the end of the day if you are able to relieve the decision-making process by changing things up in the middle of the day. Take a break. Let the decisions wait.
Even fifteen minutes can refresh you. Also, make certain you haven't skipped a meal. Maintaining steady blood sugar levels will protect you from decision fatigue as well. (You already know this, if you've ever tried to go grocery shopping on an empty stomach.)
Decision fatigue is not imaginary. You really are tired if you've been making decisions all day. Now I'm going to suggest that you have another choice to make: which decisions can you put off to tomorrow, in the interest of knowing what to buy at the supermarket this evening? and in the interest of peace of mind?