When you have an important test coming up, it's natural to assume that you would be better off

staying up late to study than going to bed. Unfortunately, this assumption is wrong, and can have a negative effect on test scores. Getting enough sleep before a test is crucial for two reasons.
First, sleep helps you synthesise and remember the information that you have learned during a study session. Without sleep, that information will not be retained. So it is possible that you will forget it by the time you actually take the test.
Second, sleep plays a role in concentration, focus, and alertness. It's difficult to think clearly and focus on tasks, and thought processes seem to slow down.
This is obviously a bad combination if you have to take a test or exam. Students who get adequate sleep perform better academically. When it comes to choosing between staying up all night to study and going to bed, getting adequate sleep is usually the better option.
Finally, it's worth noting that the effects of sleep deprivation can last for several days. That means if you stayed up most of the night on Tuesday, the effects may linger until Thursday or Friday. Therefore, it is important to develop a routine that allows you to consistently get seven to eight hours of sleep each night.

Adopted from TestTakingCentral.com