Yes, Scope of negation.In English grammar books, this issue is known as the scope of negation - ie which bit is being negated. It's usually possible to move the not about so that it's clearer which bit it is, but I don't think that's possible in your sentence, so both meanings are possible, as olwman says. The difference would probably be signalled in speech though. For the 'study some of the day' meaning I would expect all to be stressed in speech (to emphasise that that is what is being negated). In writing, it might mean italicising all. For the 'not study at all' meaning, I would expect normal stress (ie on STUDY and DAY).