It starts simple.

5 hours every Monday, Wednesday, and Friday.

But then comes the homework- Alchemy is a 6th Year course and Snape tends to overwork those he respects.

Suddenly she's turning Tuesdays and Thursdays.

She drops Muggle Studies and Divination; confident in her ability to self-study for their OWLs. But then Snape gives her access to the Library's Alchemy section and her Saturdays are shot. (It's Alchemy!)

By the time Sirius makes his first move she's so sleep deprived she's hallucinating.

That's her excuse and she's sticking to it.

Sleep deprivation.