We all know that insufficient sleep takes a toll on our mental wellbeing: it can make us drowsy, unmotivated, and even moody at times.

But beyond the generally low feelings that sleepiness brings on, there are real negative consequences to our ability to think clearly, react quickly, and make smart decisions.

One of the reasons this happens is that when we are sleepy, our brains respond more slowly to events and information in the environment. Scientists can see this in the laboratory when they test both well-rested and sleep-deprived people on different tasks that require quick thinking. In these tests, people who have missed hours of sleep have slower reaction times (sometimes drastically slower), than their rested counterparts. Tired people's brains take longer to recognize information, process it, and turn it into a response.

When you're tired, you may feel as though you're thinking just as quickly, but even split-second differences in reaction times can make a big difference—especially if you're driving, taking care of a child, or operating machinery at work, for example.

We also make more errors when we're sleepy, because the brain misses cues and has lapses in judgment. In fact, scientists have seen that sleepiness can be as bad or worse for a person's reaction time, accuracy, and judgment than a blood alcohol level of .05 or even .1 percent.

Even if you stay safe when you're tired, your mind is not as smart or creative. This is in part because you need sleep to retain information. But it's also because sleep itself builds new connections between pieces of information. After you learn something, a good night's sleep helps you make associations and bring those new facts and skills together. In other words, being tired drains your mental resources—one of the benefits of healthy sleep is having good, fresh, creative ideas.