None, unless you're also recalling high. Well, unless you're so inattentive that you just can't study at all while sober.

Take a look at something called the State Dependency Effect.

In the 70s they did some research on this. Subjects were either sober or high while studying, and sober or high while recalling. Four different groups and the results were pretty telling. If I recall correctly, the sober studying / sober recalling group had a 23% recall rate. High / High had a 21% recall rate - very close. High study / sober recall had a 17% recall rate. And sober / High had a 15% recall rate. That's a significant drop.

One of the reasons that I'm excited about legalization in Canada is the studies that can now be legally done.

You can also look up a paper called “marijuana and memory impairment” from 1977.