She looks sleepy. She might have stayed up late last night.
Literal
She [topic-は] sleepy face [object-を] is-doing. Last-night [topic-は] staying-up-late did [might-のかもしれない].
A two-sentence observation and inference. 眠そうな顔をしている ('is making a sleepy-looking face') uses そうな for appearance. のかもしれない combines explanatory の with possibility かもしれない to speculate about the cause.