It is all about the best time to go to bed if you want 8 hours of sleep. Dreams are composed of memories and mental health which get jumbled together in your sleep, making surreal associations that your wake mind would not do about their daily experiences and preoccupations.
People who have experienced trauma have nightmares which are longer well remembered, and characterized by emotions by fear dreams.
Nightmares are different. Feel the emotion as much as possible. Having better dreams with better sleep, to improve your dreams, make a decision to remember in the morning, make a note of them as much as you remember.