Sleep paralysis is relatively common and mostly harmless, although it can be very frightening. It occurs when the muscle paralysis of REM sleep persists on waking. It may be accompanied by a sense of being crushed – or having someone sitting on the chest. This is caused by the paralysis of the intercostal muscles making it difficult to expand the ribcage fully like when breathing in deeply when awake. Understandably, the person usually panics when they realise they cannot move and tries to take a large intake of breath, but finds they cannot fully do this. Sleep paralysis can sometimes be accompanied by other unusual experiences or hallucinations. These brief hallucinations on waking are called hypnopompic hallucinations, they are caused by an incomplete sleep state transition, and are not considered a sign of any underlying psychiatric illness.
For many, understanding the mechanisms of sleep paralysis, particularly how the feeling of pressure on the chest is caused, can make the experience less frightening, as there is less cause for panic.
Throughout the ages various mythology and folklore has been created to explain sleep paralysis, and (often quite macabre) artwork continues to be inspired by this experience. Here is a site which includes artistic and cultural references to sleep paralysis as well as scientific information: http://www.thesleepparalysisproject.org/.