Depersonalisation is both a symptom and an illness.
It was first described by Ludovic Dugas, a French Psychiatrist writing at the turn of the century. It occurs in almost all the major psychiatric disorders including severe anxiety, panic disorder, depression, post-traumatic stress disorder, obsessional compulsive disorder, schizophrenia, as well as neurological conditions such as migraine and epilepsy. Normal peoples can experience it during states of fatigue, fear or meditation, or after ingestion of drugs such as cannabis and 'Ecstasy'. When such unpleasant feelings persist without explanation, the person may be judged to be suffering from depersonalisation disorder. It can be brought on by severe stress or emotional turmoil but may also appear out of the blue, and apparently suddenly.