A nightmare is, in its most basic essence, a frightening dream. They cause extreme negative emotional responses such as: terror, horror, sadness, anxiety and/or despair. When someone dreams, they are actually quite active; blood pressure and adrenaline rise, while the dreamer's heart beats faster. After having a nightmare many people have problems returning to sleep.
Most nightmares occur during REM (rapid eye movement) sleep, which is the time when most dreaming occurs.
REM is the fourth stage of sleep, and people spend about 20% of their time sleeping in this stage. It can take 30 to 90 minutes after falling asleep to start dreaming. Even though someone in REM is experiencing shifts in heart rate and blood pressure, they are immobile, a condition called "REM Paralysis." While the brain is just as active during sleep as it is awake, there are chemicals missing that are normally present during waking hours. While sleeping Noradrenaline and serotonin are absent; these chemicals are involved in memory, and because they are not present, most people have problems remembering dreams.
There are several things that can induce nightmares, they can be due to either psychological or physical causes. Many people have nightmares after eating right before bedtime; eating increases metabolism and makes the brain more active. Many prescription antidepressants are known to cause nightmares due to the way that they affect changes to brain chemistry. People going through withdrawals also have an increased frequency of nightmares, again probably due to changes in brain chemistry. Additionally, sleep cycle problems can cause nightmares. People with sleep apnea and restless leg syndrome have a higher incidence of nightmares than those without sleep disorders.
Psychological reasons also exist for nightmares; stress, anxiety, and depression are common causes of nightmares and night terrors. Many times nightmares will mimic psychological issues that the dreamer is experiencing during waking hours. Dreams where people are being chased may indicate that in their lives they feel they are trapped in a bad situation.
Nightmares can be fall out from problems in the dreamer's life, or merely a physical manifestation of too much pizza before bed time.