Shadow people are black mass, shadow being or shadows believed to be spirit or entity by paranormal believers. They are thought to choke people in sleep however they repel by taking name of god.
Some of religious and paranormal organizations believe they are ghosts or supernatural beings. But some people believe they are not ghosts, evil spirits or supernatural beings but they are people of other dimension.
The scientific reason behind this is thought to be sleep paralysis and other theory also says it is thought to be because of sleep deprivation. Due to these conditions hallucinations may occur.
Many people have also claimed to have recorded shadow person in video. It has been included in many haunted shows and also the inspiration for 2013 movie Shadow People.