Your brain has two systems for processing the stimuli and experiences of your life and determining how you act upon them.

Conscious: The processing system you’re aware of is called System 2. It is logical and intentional and sometimes referred to as “true reasoning.” (A formal outline is a good example.) It is also slow, limited, and easily depleted. It processes about 40 bits of information at a time.

Unconscious: The processing system you’re not aware of is called System 1. It is associative, which means it sees patterns and connects dots. (A mindmap is a good example.) It is fast, vast, and always on. It processes about 11,000,000 bits of information at a time.

If System 1 were to go offline, you would, too. Game over! But you can still function when System 2 is temporarily offline, even for long periods of time, such as when you’re asleep. So when you think or talk about being temporarily brain dead, you’re talking about exhausting System 2 attention.

If you’re in good health, there’s not much you can do to tax or exhaust the capacity of System 1—and there are things you can do to enhance its functioning. However, your supply of System 2 attention is always limited, and anything that occupies your working memory reduces it. Some examples of things that tax System 2 attention are:

• Physical illness (even minor), injury, or lack of sleep
• Making numerous trivial decisions throughout the day
• Stress, anxiety, and worry
• Exercising will power (forcing yourself to do something you don’t want to do or to not do something you do want to do)
• Monitoring your behavior
• Monitoring your environment if it is new or you consider it unsafe
• Learning something new, traveling an unfamiliar route, etc.
• Completing a complex computation
• Trying to tune out distractions
• A long period of concentrated or focused attention
• Trying to remember dates, numbers, or unrelated facts
• Listening to me talk

Since System 1 is fast, vast, always on, and has an answer for almost everything—and since you don’t need System 2 attention for most of what you do when you’re awake—what’s the big deal if you run out of System 2 attention from time to time?

Three Categories of Errors

Optimally, the two systems work together, and neither type of processing is superior. However, System 1 is more useful in some situations, while System 2 is not only more useful but also required in other situations.

System 1 is pretty good at what it does because its models of familiar situations are accurate so its short-term predictions tend to be accurate. But that’s not always the case. System 1 sacrifices accuracy for speed, meaning it jumps to conclusions. It also has biases and is prone to making logical errors.

One of System 2’s jobs is to detect System 1’s errors and adjust course by overriding System 1’s impulses. As Daniel Kahneman says in Thinking, Fast and Slow:

There are vital tasks that only System 2 can perform because they require effort and acts of self-control in which the intuitions and impulses of System 1 are overcome.

Bear in mind that System 1 is not rational. If System 2 is depleted and can’t veto or modify the non-rational impulses of System 1, those impulses then turn into actions (or speech).

There are three categories of errors you tend to make when System 2 is depleted.

Logical Errors

System 1 thinking uses shortcuts. System 2 thinking takes the long (logical/linear) way home. So when you’re out of System 2 attention, you’re more prone to making mistakes in anything that requires logical, linear thinking. Errors of intuitive thought can be difficult for System 2 to catch on a good day. When System 2 is offline, you automatically assume them to be correct. As a result:

• You will have trouble making, following, or checking the validity of a complex logical argument. You’ll be more likely to be led by the cognitive biases and distortions System 1 uses because they don’t require any effort and give you a comforting sense of cognitive ease.

• You will have difficulty comparing the features of two items for overall value. If you have to make a choice, you’ll be more likely to go with what intuitively feels right or the item that has some emotionally compelling attribute (it reminds you of the one your mother had, for example, or reminds you of your mother).

• You will be more gullible. You’ll be more likely to believe things you wouldn’t otherwise believe or be persuaded by empty messages, such as in commercials. System 2 is the skeptic, so the best time for someone to take advantage of you is when it is offline.

Intention or Response Errors

System 1 continuously picks up on cues and triggers in your environment to determine what situation you’re in and to predict what’s next. Any deviation from the norm requires System 2 attention. If it isn’t available, you’re likely to do not what you intended to do but whatever is normal for you in that situation. And without System 2 attention, you’re much more likely to respond automatically (habitually) to the stimulus (cue or trigger).

• System 2 is in charge of self-control, continuously monitoring your behavior, keeping you polite, for example, when you’re angry. In the heat of the moment, when you’re out of System 2 attention, you’re much less likely to be able to suppress your immediate emotional reactions to people and situations.

• System 1 has an answer for almost everything. But when it encounters a surprising situation (something it hasn’t previously encountered or that is unusual in that situation), it notifies System 2. You don’t need System 2 attention to drive a familiar route, but if you encounter an obstacle along that route, you need System 2 to figure out what it is and to respond appropriately to it.

• System 2 is also in charge of will power. If you are in the process of trying to stop doing something you habitually do (such as raiding the refrigerator in the evening), you need System 2 to belay the impulse from System 1 to see if there’s more pie. Without System 2, you’re more likely to give in, look for the pie…and eat it.

• You need System 2 if you want to take a different route from your usual one or make an extra stop you don’t normally make. Without adequate System 2 attention, you’re likely to find yourself taking the usual route and forgetting to make that stop.

Gatekeeping Errors

We all have biases, whether or not we’re aware of them and whether or not we want to admit it. While it’s easy to spot overt biases and prejudices in other people, most of your own biases are hidden even from you. In the case of biases toward specific groups of people, you’ve likely come to a reasoned conclusion they’re wrong and have chosen not to think about and treat other people based on stereotypes. But that doesn’t mean the biases have disappeared. They’re still part of System 1’s associative processing operations. It’s just that when System 1 suggests a biased response to System 2, System 2 normally overrides it. Per Daniel Kahneman:

Conflict between an automatic reaction (System 1) and an intention to control it (System 2) is common in our lives.

When System 2 is depleted, there is no one at the gate to keep the biased or prejudiced responses from getting through. You may simply have a biased thought. You may say something in the presence of others that you wouldn’t normally say. Or you may respond to another person based on a group stereotype. The thought, comment, or behavior may be something you later regret. If you were to claim it doesn’t represent what you believe or the way you really feel or think, you’d most likely be right.

But when you see a blatant expression of bias or prejudice in someone else—especially a celebrity—you might have a different reaction. You might assume their true colors are showing.  We think that what we see in other people when their guard is down and they’re pushed or stressed reveals the truth about them. But the actual truth is that to the extent we have any civility at all, it’s because System 2 maintains it.  Without System 2 you and I would have no ability to question our biases or prejudices, no ability to come to reasoned conclusions about them, and no ability to monitor and veto System 1’s automatic reactions.

Conclusion

It isn’t always necessary, advisable, or even possible to override System 1. But when you deplete System 2, you can’t override it even when you want or need to. Without System 2, you can’t think straight (logically and linearly). So:

• Don’t try to make important decisions of any kind when you feel brain dead.
• Don’t assume you’ll feel or think the same way about something the next day as you do when you’re stressed, sick, just completed your annual tax return, or have recently fallen in love.
• Don’t stay up late to watch the QVC channel unless you have a lot of money you’re trying to unload.
• Don’t keep pie around if you’re trying not to eat it.
• Don’t get into debates about complex issues after you’ve had a few beers.
• Don’t tax your working memory with details you can keep track of some other way.
• Don’t take System 2’s censoring of your biases and prejudices for granted. And don’t assume other people’s mental lapses reveal deep-seated truths about them.