The most eye-opening thing I read in Willpower: Rediscovering the Greatest Human Strength, by Roy Baumeister and John Tierney, is what the authors call "Decision Fatigue". They describe a wide variety of research showing that people get tired when they make many decisions without a break, quickly losing the ability to make further decisions wisely and fairly. In one incredible case, a variety of judges heard parole requests all day long. On average, if they heard a case early in the morning, they granted parole 65% of the time. But the judges were much harder on the poor prisoners who had the bad luck to appear late in the day. The judges paroled them only 10% of the time even though their crimes and their sentences were no different than those who appeared in the morning.
Turns out that even small decisions of minor importance are draining. A series of shopping decisions, for example, can drain you of the mental resources needed to make harder decisions later.
Walk into an automobile showroom and WATCH OUT. They'll have you make a ton of trivial decisions about trim, the color of the dashboard, color of the carpets, options for cup holders and then WHAM! They ask if you want the $3,000 V6 Turbocharger option, knowing that you'll just say, "Whatever!"
Makes me think about conserving my decisions. Drive in the middle lane. In the left lane, I have to constantly decide whether the guy behind me is coming up so fast that I'd better move over. In the right lane, should I speed up or slow down so the Ford Pinto can merge? In the middle, just go with the flow. Similarly, lock in the cruise control 5 mph above the speed limit and forget it. No peering ahead for the cop car, ready to make a decision to tap the brakes. Maybe that's why lots of people eventually start to drive this way. They start to see how draining these small decisions can be on a long drive. Better to save the energy for whatever I find when I get to the destination.