After a single drink, you think: I can still drive safely.

After two or three drinks, you think: I’m getting drunk, I probably shouldn’t drive.

After four or five drinks, you think: wait, I can totally still drive!

Obviously, five drinks in, you’re drunk as a skunk. But your ability to perceive that has declined alongside all of your other cognitive abilities. That’s why, despite all of the national campaigns against the real danger of driving drunk, about 20% of US drivers do it every year.

Our brains lie to us all the time, in large part because they so quickly adapt to novel stimuli. What at first seems out of the ordinary quickly becomes the new normal. And once we acclimate, our subjective self-evaluations are basically crap.

Consider something many of us do, like multi-tasking. Perhaps you tell yourself that you’re good at it, that you’re practiced at flitting seamlessly from one focus to the next, that you can effectively handle multiple things at the same time.

Turns out, you’re wrong. Not only has a growing body of research shown that we’re terrible multitaskers, it’s also shown that we’re terrible at knowing how bad we are at it. In one great Stanford study, researchers even found that the better people thought they were at multitasking, the worse they actually performed on multitasked tests.

Or consider sleep, where many of us have convinced ourselves that we can get by fine on less than seven or eight hours a night.

In another great study, researchers took well-rested subjects, then reduced their sleep from eight to six hours a night. After a single sleepless night, the subjects performed less well on cognitive tasks, but they also knew that their performance had declined. In fact, for the first three days, performance continued to drop daily, though subjects also correctly assessed their worsening mental states.

But by the fourth day, things changed. Subjects didn’t get better at the tests; in fact, from that point on, performance continued to drop. But people completely lost the ability to tell that it did. They reported to researchers that they felt fine, believing that they’d returned to solid performance, even as their test results continued to go down the toilet.

So what’s the moral of the story? If you can’t trust your brain, you need to trust objective measures instead. Testing blood alcohol content, or even just counting drinks, is more useful than seeing if you feel buzzed. Start tracking the number of hours you sleep. Do workouts where you can measure performance changes over time. Log the time you spend on deep, focused work versus responding to emails and wrangling small, seemingly urgent tasks. In short, start quantifying yourself; it’s the only way to know what’s really happening in your life.