If you've ever said, "I can't even remember the last time I got a good night's sleep," your words might be more revealing than you think. After a restless night, you may become forgetful or experience "senior moments."
Research has shown that deep sleep plays an important role in memory, because it facilitates connections between nerve cells [source: The Franklin Institute]. The less deep sleep you get, the fewer connections form between nerve cells. Both humans and animals perform worse on memory tests when deprived of sleep.
REM sleep especially aids in the formation and retention of emotional memories, which tend to be those that are most vivid to us decades down the road.
So what's the upshot? If you're facing a task that will call heavily upon a sharp memory, you may want to choose sleep over late-night preparation. For instance, a student during "finals week" shouldn't pull an all-nighter. It's better to study until feeling tired and then get plenty of sleep before the next day's test.
If you read that last sentence at least twice, you may have another common sign of sleep deprivation, which we'll discuss in the next section.