[theorique]

Competing with 'average' high school students on 'average' high school material and still coming out on top is easy for many highly gifted people (indeed, that's sort of the definition).

As you get more advanced and specialized in your level of education, the material gets harder and the peer group gets more focused on a narrower area or field.

Even if grading is not done on a formal curve comparing students to each other, a test aimed at graduate students' skill level is going to assume a greater level of baseline knowledge, and more facility with techniques and tools of the field. So it will take more of even a 'gifted' person's time to perform far above average at graduate school.

The sleep disruption thing is another possible explanation. Another comment mentioned sleep apnea. You might want to get that checked out.