A nightmare is a disturbing and distressing dream that typically causes feelings of fear, anxiety, dread, or despair, and often results in the sleeper waking up abruptly. Upon waking from a nightmare, the person usually has a clear and vivid memory of the dream content and may experience lingering emotional distress, a racing heart, or difficulty returning to sleep. Nightmares occur most commonly during REM (rapid eye movement) sleep, the stage of sleep associated with vivid dreaming, which becomes more prevalent in the second half of the night. This is why nightmares often occur in the early morning hours. Occasional nightmares are a normal part of human experience and can be triggered by stress, anxiety, trauma, illness, or even certain foods or medications. However, when nightmares are frequent, severe, or significantly affect sleep quality or daytime wellbeing, they may indicate a condition known as nightmare disorder or may be associated with post-traumatic stress disorder (PTSD), anxiety disorders, or depression. Children experience nightmares more frequently than adults, and for most, the frequency decreases with age. Nightmares triggered by traumatic experiences may persist into adulthood and can be a central feature of PTSD. Effective treatments for nightmare disorder include Image Rehearsal Therapy (IRT), a cognitive behavioral technique in which the person imagines and rewrites the content of recurring nightmares during the day, changing the ending to something less distressing. Other approaches include therapy for underlying anxiety or PTSD, and in some cases medication. Practicing good sleep hygiene, managing stress, and limiting alcohol and caffeine can also help reduce nightmare frequency.