Nearly everyone has nightmares occasionally, and that's considered a normal part of healthy dreaming. Nightmare disorder is a distinct, recognised clinical condition — when nightmares become frequent enough, and distressing enough, to genuinely disrupt sleep and daily functioning.
What separates a bad dream from nightmare disorder
The frequency and impact are what matter, not just the content. Clinically, nightmare disorder is generally considered when nightmares occur regularly (often at least once a week over an extended period), cause significant distress, lead to fear of falling asleep, or noticeably affect daytime mood, concentration, or functioning. A single terrifying dream after a stressful week doesn't meet this bar; a pattern that's actively shaping how you feel about going to bed does.
What causes it
Nightmare disorder can occur on its own, but it's also strongly associated with anxiety disorders, depression, and — very commonly — PTSD, where recurring nightmares that replay or reference a traumatic event are a recognised diagnostic symptom, distinct from ordinary anxiety dreams. Certain medications and alcohol use can also increase nightmare frequency and intensity.
Why "just ignore it" doesn't work
Frequent nightmares create a genuine feedback loop: fear of nightmares causes anxiety about sleep, which increases sleep-onset difficulty and stress, which in turn increases the likelihood of more nightmares. Breaking that loop usually requires more than willpower or simply trying not to think about it before bed.