Devar Eliyahu: Nevu'ah comes via two matters. (a) Taharah of thought and great preparation. (b) Separation and meditation. However, people who have [visions in] dreams, their preparation is in thinking about the dream; when it comes, that is their meditation. We do not find like this in Nevi'im.

Malbim: The third shortcoming - it came in a dream at night. It was not at the end of the night, when his thoughts of the day ceased, rather, when thoughts are still full of night visions, at the start of sleep.