(Harry Potter) Pillow Thought

An essay I read once mentioned something called a clear conscience on the pillow, or something like that. The idea was that when you suddenly wake in the night, many things ignored or suppressed during the day awaken with you, allowing you to see your life as objectively and truthfully as possible. A moment of regret or sadness follows, but by daylight everything is back to normal. The timeline is roughly one night in the summer before Half-Blood Prince. After Bellatrix kills Sirius Black, she discovers a certain chemistry between them... The author has no idea how that happened, either. The characters and world belong to Rowling. Theme: