Imagine that a student studies all night for an exam the next day. The student remembers (almost) everything for her test, but within a week she forgets most of what she studied. Why?

She crammed for the exam

Explanation:

Cramming is a method of studying, whereby a student tries to memorize enough information about a subject within a short period of time, and variably after the exam or test, they tend to forget most of the information they studied within that period.

This method is for a period of time, and as it is stated in the question, the student studied all night for the exam the next day, such student studying because of the exam the next day shows that he/she didn't have enough time to study before the night, so the only time they to study is all night, the can of study such student will engage in is cramming, and that is way after a wee, such student could not remember most of what she studied because she didn't study, she crammed for the next day exam.