Both nightmares and night terrors are considered "parasomnias" in the field of sleep medicine, explains Dr. Raj Dasgupta, MD, a fellow of the American Academy of Sleep Medicine and assistant professor of clinical medicine at the Keck School of Medicine at the University of Southern California. Basically, they're unwanted actions or thoughts that occur as you’re falling asleep, waking up, or transitioning into a different state of sleep.
A nightmare elicits an emotional response. While most people think of fear when they think of nightmares, "people don't realize that other classic emotions that people feel — anxiety, disparity, sadness — are real nightmares, too," Dr. Dasgupta says. Nightmares that leave you feeling shaken, nervous, or sad can be just as much, if not more, upsetting than a nightmare filled with more traditional monsters, and can often reflect what’s going on in your waking life. But don’t be alarmed if you can't seem to link your nightmare with something that happened in the last 24 hours — according to Dr. Dasgupta, a dream is the result of the past several weeks, rather than just the last few days, and may be a reflection of stress you haven’t even thought about that day.