Just as in waking life, your body has reacted to frightening images by pumping adrenaline.

ur whole body reacts to what ur brains thinks I guess even when u r asleep

Nightmares usually occur during the second half of the night's sleep. During the course of a nightmare the sleeper may moan, talk, or move slightly, although these signs do not always appear. The person wakes from the nightmare with a profound sense of fear. Waking is complete, and usually accompanied by increased heart rate, "sweating", and other symptoms of anxiety or fear. Sometimes people may avoid going to sleep after a particularly intense nightmare because of the fear of having another bad dream. In addition, people may have problems falling asleep if they are experiencing anxiety caused by the fear of having nightmares. As a result, these people may have the signs and symptoms associated with mild sleep deprivation, such as decreased mental clarity, problems paying attention, excessive daytime sleepiness, irritability, or mild depression. Nightmares can be a side effect of some medications or drugs of abuse, including drugs given for high blood pressure; levodopa and other drugs given to treat Parkinson's disease; amphetamines, cocaine, and other stimulants; and some antidepressants. Withdrawal from alcohol and other medications can also sometimes cause nightmares. http://www.reference.com/search?q=nightmare

For a long time, due to the ignorance of physiological knowledge of ischemic heart, slow beat, fast beat and so on, psychological illusion in people’s sleep generated by such physical symptoms i.e. the nightmare really has puzzled the psychologists, therefore they put forward a wide range of wrong even absurd views on the nightmares, which both have no scientific basis, and could not be confirmed, even more were not self-consistent. A person with a heart disorder ever had a nightmare of falling from high, and he conceived it was a fierce boding, so he had no peace of mind day and night, unexpectedly, at last he jumped from a 11-Story building to his death. A fugitive murderer, fled in two years, he surrendered to the police because of the psychological stress due to the unbearable illusion of as if being hunted all day long, and had nightmare all the night that the slain captured the fetch. In fact, these two nightmares are caused by premature beat or slow beat or fast beat, and they are “One dreams at qnight what one thinks in the day” The reasons for the dreams are very simple, viz. they are the reactions of the objective things in the minds. As Germany's well-known psychologist William Wundt’s view of “the most common causes of the dream are indigestion, heart pulsation (palpitation or flustered), difficulty in breathing, as well as such symptoms." Psychologist A Moree after a rigorous study also pointed out that: "The dream is the organism having a misunderstanding of the emotional perception, for example, the noisy at night when sleep will cause a dream of sound of thunder. For instance, students dreaming the examination before the pre-examination, is caused by the things about examination, and dreams about urgency of urine are caused by the sense of urgency of urine after sleep. By the same token, the nightmare is caused by the sense of hear throb such as fast beat, slow beat. For instant, if a certain heartbeat occurs in frequency in the night, it will cause phenomena of insomnia with iterative dreamlands. The heart throb in nightmare will be stronger than daytime, which is the same as the phenomena that early in the disease, some people can not feel the slight discomfort during daytime, but they can feel it in the dream, because in sleep, the incitements that the brain receiving from the outside world are largely reduced, so the incitements from inner become comparatively stronger. When palpitation or flustered occurs, people will have two main feelings: one is tachycardia cause a feeling of seeming to be pursuing (TCM: tachycardia, watchful as if he was being pursuing); the other is bradycardia or premature beat cause the feeling of heart suspension or heart sinking. Therefore, people in sleep accordingly will have the two nightmares of being pursued (occasionally hunting other people) and hanging in the air, and falling down. If transient cerebral ischemia occuring during the day may result in some very terrible dizziness, palpitations, feelings of chest pressure, dim vision, tinnitus and a variety of neurological symptoms. As a result, all the people who are prone to cerebral ischemia frail corporeity, excessive fear, taking the quinidine which may lead to low blood pressure, as well as a excessive high pillow ors, error in sleeping style which may lead to the aggrieved neck, pressed blood stream, when they sleep in deep night, they will have the extraordinary corresponding horrible dreams, in the dreams, they do some kind of terrible Belial pressuring them or being hunted down, but they can not cry out or escape, which are called nightmare in iatrology. Vague terrors in light sleep, which is known as sleep paralysis. Sometimes people was just woken up with the cerebral ischemia, be cause the vision continuing for a few minutes and dyskinesia have not yet concluded, which will cause psychological illusion that people struggle to wake up but fail to do it. For instance, a place in country, there is a "haunted" bed which makes people have nightmares every night, and it is this fact that the pillow in the bed is too high. Psychological distress during the day will cause being frightened due to a very small voice. In the cause of insomnia which due to the mental stress, and changing the sleep style constantly, people sometimes suddenly as if heard a loud noisy them have the nightmare. It is obvious that the loud noisy is generated by the friction in the process of changing the sleep style. In normal times, such a "bang" which has turned a deaf ear to the people, sometimes will become very terrible when mental stress. Another example, sleeping pills are the treatment of nightmare of neurasthenia, due to the nightmare of such patients is caused by tachycardia, and sleeping pills can lead to slower heartbeat, moreover slower heartbeat sometimes leads to cerebral ischemia for people of low blood pressure. Therefore, treating a nightmare with sleeping pills at times is not only invalid, but also it will instead increase the patient's state of the disease. 　 In short, all of the things are experienced in the dreams are the experienced things which are in people’s minds during the day. No one will feel afeared and mysterious because he suddenly thinks of a person who had been dead for many years during the day, similarly, no one will feel mysterious and specially afeared because of low beat or fast beat, likewise there is no need to feel mysterious and specially afeared because of the sudden nightmare due to low beat or fast beat.

when you lay down horizontally it throws off the digestion mechanism. The uncomfort of indegestion can manifest in nightmares. Additionally eating generally causes increased blood flow thoughout the body especially the abdomen to deliver the ingested nutrients. This can in turn cause the brian to remain more active than normal when sleeping is you eat before u sleep, or it could be because you are suffering from stress or cant let go of a bad memory, if you sweat, wake up suddenly or shout this is called pavor nocturnus you should refer to your gp (doctor) lawrence ribton you can talk to me by emailing me at lawrenceribton@hotmail.co.uk if you have any questions or queries.