The importance of sleep is often overlooked by students when they are preparing for exams. Research has consistently shown that sleep deprivation greatly reduces the ability to recall the material learnt. Hence, cutting down on sleep to study longer hours can be counterproductive. Which one of the following statements is the CORRECT inference from the above passage?

The correct option is: To do well in an exam, adequate sleep must be part of the preparation.

Let us break down the logical reasoning of the passage step-by-step:
1. Premise 1: Students preparing for exams often overlook the importance of sleep.
2. Premise 2: Research has shown that sleep deprivation greatly reduces the ability to recall the material that has been learned.
3. Conclusion: Because of the reduced ability to recall material, cutting down on sleep in order to study longer is counterproductive (meaning it hinders performance rather than helping it).
4. Inference: Since recalling learned material is essential to performing well on an exam, and adequate sleep is required to maintain recall ability, it logically follows that adequate sleep must be an essential part of exam preparation.

Let us evaluate why the other options are incorrect:
- "Sleeping well alone is enough to prepare for an exam. Studying has lesser benefit.": This is incorrect because the passage does not suggest that studying is unnecessary or of lesser benefit; it only states that sacrificing sleep to study longer is counterproductive. Both are necessary.
- "Students are efficient and are not wrong in thinking that sleep is a waste of time": This directly contradicts the passage, which argues that students are indeed wrong to overlook sleep, as doing so reduces their learning efficiency.
- "If a student is extremely well prepared for an exam, he needs little or no sleep.": This is incorrect because the passage states that sleep deprivation reduces the ability to recall learned material, regardless of how well-prepared the student is.