The idea that spirits or supernatural entities manifest during dreams or nightmares is rooted in various cultural beliefs and folklore. Many people believe that during sleep, especially in a vulnerable state, individuals become more open to experiencing paranormal phenomena.
Some theories suggest that when people are not covered or protected (like using blankets, which can symbolize a barrier), they may feel more exposed to the unknown, which could lead to more intense or frightening dreams. This isn't universally accepted, however, and many interpretations of dreams and nightmares focus on psychological factors rather than supernatural ones.
Ultimately, the experience of spirits in dreams or nightmares is subjective and heavily influenced by personal beliefs, cultural backgrounds, and individual psychology. If you're interested in this topic, exploring folklore, dream interpretation, and psychological perspectives on fear and vulnerability may provide deeper insights.