There are many people in the world who are classified as being 'mentally sick'. That is, they suffer from ailments that are called schizophrenia (a long-term mental disorder of a type involving a breakdown in the relation between thought, emotion, and behaviour, leading to faulty perception), melancholia (type of sadness), neurosis (a relatively mild mental illness that is not caused by organic disease, involving symptoms of stress- depression, anxiety, obsessive behaviour, hypochondria), etc. Actually, it is a matter of degree, for we are all mentally sick, some less than others. Mental sickness, indicating disharmony in the mind no matter how small, only disappears with the onset of self-realization.

The cause of mental sickness is disorder in the mind, perhaps in the form of fears, conflicts etc. These can be systematically and progressively released during yoga nidra practice. During sufficient deep states of relaxation one starts to confront visions, subconscious memories, childhood traumas and so forth. This process can also be called conscious or yogic dreaming. During the practice, you may see demons, ghosts but mostly things stranger to you. Sometimes pleasant, sometimes very unpleasant, they represent the conflicts of your mind. You have to be a witness. Feel that these visions are separate from yourself. In this way, they will not be re-suppressed.

The mind gradually becomes free of these emotionally loaded subconscious knots and becomes more relaxed, then mental sickness gradually disappears like a ghost in the night.
Yoga nidra can also be used to eliminate psychosomatic diseases. Yoga nidra exposes all complexes. It removes all mental knots. It can be used by psychiatrists with patients.