dreaming. studies comparing in-dream sex, arousal, and orgasm. communication between two dreamers has also been documented. the processes involved included eeg monitoring, ocular signaling, incorporation of reality in the form of red light stimuli and a coordinating website. the website tracked when both dreamers were dreaming and sent the stimulus to one of the dreamers where it was incorporated into the dream. this dreamer, upon becoming lucid, signaled with eye movements; this was detected by the website whereupon the stimulus was sent to the second dreamer, invoking incorporation into that dreamer's dream. recollection raphael's dream (1821). johannes riepenhausen and franz riepenhausen. the recollection of dreams is extremely unreliable, though it is a skill that can be trained. dreams can usually be recalled if a person is awakened while dreaming. women tend to have more frequent dream recall than men. dreams that are difficult to recall may be characterized by relatively little affect, and factors such as salience, arousal, and interference play a role in dream recall. often, a dream may be recalled upon viewing or hearing a random trigger or stimulus. the salience hypothesis proposes that dream content that is salient, that is, novel, intense, or unusual, is more easily remembered. there is considerable evidence that vivid, intense, or unusual dream content is more frequently recalled. a dream journal can be used to assist dream recall, for personal interest or psychotherapy purposes. adults report remembering around two dreams per week, on average. unless a dream is particularly vivid and if one wakes during or immediately after it, the content of the dream is typically not remembered. recording or reconstructing dreams may one day assist with dream recall. using the permitted non-invasive technologies, functional magnetic resonance imaging (fmri) and electromyography (emg), researchers have been able to identify basic dream imagery, dream speech activity and dream motor behavior (such as walking and hand movements). in line with the salience hypothesis, there is considerable evidence that people who have more vivid, intense or unusual dreams show better recall. there is evidence that continuity of consciousness is related to recall. specifically, people who have vivid and unusual experiences during the day tend to have more memorable dream content and hence better dream recall. people who score high on measures of personality traits associated with creativity, imagination, and fantasy, such as openness to experience, daydreaming, fantasy proneness, absorption, and hypnotic susceptibility, tend to show more frequent dream recall. there is also evidence for continuity between the bizarre aspects of dreaming and waking experience. that is, people who report more bizarre experiences during the day, such as people high in schizotypy (psychosis proneness), have more frequent dream recall and also report more frequent nightmares. miscellany illusion of reality some philosophers have proposed that what we think of as the "real world" could be or is an illusion (an idea known as the skeptical hypothesis about ontology). the first recorded mention of the idea was in the 4th century bce by zhuangzi, and in eastern philosophy, the problem has been named the "zhuangzi paradox." he who dreams of drinking wine may weep when morning comes; he who dreams of weeping may in the morning go off to hunt. while he is dreaming he does not know it is a dream, and in his dream he may even try to interpret a dream. only after he wakes does he know it was a dream. and someday there will be a great awakening when we know that this is all a great dream. yet the stupid believe they are awake, busily and brightly assuming they understand things, calling this man ruler, that one herdsman—how dense! confucius and you are both dreaming! and when i say you are dreaming, i am dreaming, too. words like these will be labeled the supreme swindle. yet, after ten thousand generations, a great sage may appear who will know their meaning, and it will still be as though he appeared with astonishing speed. the idea also is discussed in hindu and buddhist writings. it was formally introduced to western philosophy by descartes in the 17th century in his meditations on first philosophy. absent-minded transgression dreams of absent-minded transgression (damt) are dreams wherein the dreamer absent-mindedly performs an action that he or she has been trying to stop (one classic example is of a quitting smoker having dreams of lighting a cigarette). subjects who have had damt have reported waking with intense feelings of guilt. one study found a positive association between having these dreams and successfully stopping the behavior. non-rem dreams hypnogogic and hypnopompic dreams, dreamlike states shortly after falling asleep and shortly before awakening, and dreams during stage 2 of nrem-sleep, also occur, but are shorter than rem-dreams. daydreams dante meditating, 1852, by joseph noel paton. a daydream is a visionary fantasy, especially one of happy, pleasant thoughts, hopes or ambitions, imagined as coming to pass, and experienced while awake. there are many different types of daydreams, and there is no consistent definition amongst psychologists. the general public also uses the term for a broad variety of experiences. research by harvard psychologist deirdre barrett has found that people who experience vivid dreamlike mental images reserve the word for these, whereas many other people refer to milder imagery, realistic future planning, review of memories or just "spacing out"—i.e. one's mind going relatively blank—when they talk about "daydreaming". while daydreaming has long been derided as a lazy, non-productive pastime, it is now commonly acknowledged that daydreaming can be constructive in some contexts. there are numerous examples of people in creative or artistic careers, such as composers, novelists and filmmakers, developing new ideas through daydreaming. similarly, research scientists, mathematicians and physicists have developed new ideas by daydreaming about their subject areas. hallucination main article: hallucination a hallucination, in the broadest sense of the word, is a perception in the absence of a stimulus. in a stricter sense, hallucinations are perceptions in a conscious and awake state, in the absence of external stimuli, and have qualities of real perception, in that they are vivid, substantial, and located in external objective space. the latter definition distinguishes hallucinations from the related phenomena of dreaming, which does not involve wakefulness. nightmare main article: nightmare woman having a nightmare. jean-pierre simon (1764–1810 or 1813). a nightmare is an unpleasant dream that can cause a strong negative emotional response from the mind, typically fear or horror, but also despair, anxiety and great sadness. the dream may contain situations of danger, discomfort, psychological or physical terror. sufferers usually awaken in a state of distress and may be unable to return to sleep for a prolonged period of time. night terror main article: night terror a night terror, also known as a sleep terror or pavor nocturnus, is a parasomnia disorder that predominantly affects children, causing feelings of terror or dread. night terrors should not be confused with nightmares, which are bad dreams that cause the feeling of horror or fear. déjà vu one theory of déjà vu attributes the feeling of having previously seen or experienced something to having dreamed about a similar situation or place, and forgetting about it until one seems to be mysteriously reminded of the situation or the place while awake.