The symptoms of psychosis may be caused by serious psychiatric disorders such as schizophrenia, a number of medical illnesses, and trauma. Psychosis may also be temporary or transient, and be caused by medications or substance use disorder (substance-induced psychosis).

Normal states
Brief hallucinations are not uncommon in those without any psychiatric disease, including healthy children. Causes or triggers include:

Falling asleep and waking: hypnagogic and hypnopompic hallucinations
Bereavement, in which hallucinations of a deceased loved one are common
Severe sleep deprivation
Extreme stress 
Trauma and stress
Traumatic life events have been linked with an elevated risk of developing psychotic symptoms.[62] Childhood trauma has specifically been shown to be a predictor of adolescent and adult psychosis. Individuals with psychotic symptoms are three times more likely to have experienced childhood trauma (e.g., physical or sexual abuse, physical or emotional neglect) than those in the general population. Increased individual vulnerability toward psychosis may interact with traumatic experiences promoting an onset of future psychotic symptoms, particularly during sensitive developmental periods. Importantly, the relationship between traumatic life events and psychotic symptoms appears to be dose-dependent in which multiple traumatic life events accumulate, compounding symptom expression and severity. However, acute, stressful events can also trigger brief psychotic episodes. Trauma prevention and early intervention may be an important target for decreasing the incidence of psychotic disorders and ameliorating its effects. A healthy person could become psychotic if he is placed in an empty room with no light and sound after 15 minutes, a phenomenon known as sensory deprivation.

Neuroticism, a personality trait associated with vulnerability to stressors, is an independent predictor of the development of psychosis.