There are several medical conditions that are correlated with more frequent nightmares, as well as sleep problems more generally. Nightmares have been found to be comorbid, or to co-occur, with conditions including coronary heart disease, cancer, Parkinson’s disease, and chronic pain; they can also be a side effect of dialysis and of withdrawal from medications. In most cases, the stress surrounding the chronic illness is the likely reason that people experience more nightmares.
Nightmares are also frequently present with mental-health disorders including PTSD, insomnia, schizophrenia, psychosis, and mood, anxiety, adjustment, and personality disorders. Nightmares may also occur as one grieves during bereavement.