I [topic-は] must-sleep! Morning class [subject-が] exist [reason-から].
眠らなければなりません 'must sleep' — the full, uncontracted ~なければなりません obligation form, which sounds textbook-like. 朝クラスがあります is a straightforward reason. The sentence has a slightly stiff quality — 眠らなきゃ would be more natural in casual speech.