A psychoactive drug, psychopharmaceutical, or psychotropic is a chemical substance that crosses the blood-brain barrier. Its action affects the central nervous system. It can affect the brain and change perception, mood, consciousness, cognition and behaviour. Hypnotics are often prescribed to help people sleep. Sometimes they are also used in rituals, or as illegal drugs. These drugs enable their users to change their consciousness. This can also help students when they are preparing their examinations. Certain therapies also use such substances.