Nightmares are disturbing dreams that often cause strong negative emotions such as fear or anxiety. They typically occur during the rapid eye movement (REM) stage of sleep. The exact cause of nightmares is not definitively known, but multiple factors can trigger or contribute to their occurrence:

• Stress and anxiety: Daily life stresses, major life changes (like moving or the death of a loved one), and ongoing anxiety are common triggers.
• Trauma: Experiencing or reliving traumatic events, especially in conditions like post-traumatic stress disorder (PTSD), often leads to nightmares.
• Sleep deprivation and irregular sleep schedules: Lack of sufficient sleep or disruptions in sleep patterns can increase the likelihood of nightmares.
• Medications: Some antidepressants, blood pressure drugs, beta blockers, and medications for Parkinson’s disease or smoking cessation can cause nightmares.
• Substance misuse and withdrawal: Alcohol, recreational drugs, and withdrawal from certain medications can trigger nightmares.
• Other health conditions: Depression, heart disease, cancer, and other sleep disorders like obstructive sleep apnea are linked to nightmares.
• Psychological and genetic factors: Mental health disorders and a family history of nightmares may increase susceptibility.
• Environmental factors: Watching scary movies or reading frightening books before bed can provoke nightmares in some people.

Nightmares may also be more common during periods of fever and in people with altered sleep architecture or certain neurodegenerative diseases. In essence, nightmares are believed to be associated with the brain’s way of processing emotions, fears, and memories, particularly during REM sleep. They can serve as indicators of underlying stress, health issues, or emotional distress that the brain is trying to manage during sleep. If nightmares occur frequently or cause significant distress, they may be part of a sleep disorder known as nightmare disorder, which can require medical attention. This summary of causes is drawn from medical and sleep research sources including Mayo Clinic, Sleep Foundation, and Cleveland Clinic.