Etymology
The term 'lucid dream' was coined by Dutch author and psychiatrist Frederik van Eeden in his 1913 article A Study of Dreams,
 though descriptions of dreamers being aware that they are dreaming 
predates the actual term. Frederik Van Eeden studied his personal dreams
 since 1896. He wrote down the most interesting ones, and, out of all 
these dreams, 352 were what we know now as “lucid dreams”. Throughout 
all the data he collected from dreaming, he created different names for 
different types of dreams. He named 7 different types of dreams: initial
 dreams, pathological, ordinary dreaming, vivid dreaming, demoniacal, 
general dream-sensations, and lucid dreaming. Frederick Van Eeden said 
the seventh type of dreaming, lucid dreaming, was the most interesting 
and worthy of the most careful observation of studies. Eeden studied 
lucid dreaming between January 20, 1898, and December 26, 1912. In this 
state of dreaming Eeden explains that you are completely aware of your 
surroundings and are able to direct your actions freely, yet the sleep 
is stimulating and uninterrupted.
History
Ancient
Early references to the phenomenon are found in ancient Greek writing. For example, the philosopher Aristotle
 wrote: 'often when one is asleep, there is something in consciousness 
which declares that what then presents itself is but a dream'. Meanwhile, the physician Galen of Pergamon used lucid dreams as a form of therapy.  In addition, a letter written by Saint Augustine of Hippo in 415 AD tells the story of a dreamer, Doctor Gennadius, and refers to lucid dreaming.
Zhuangzi dreaming of a butterfly
In Eastern thought, cultivating the dreamer's ability to be aware 
that he or she is dreaming is central to both the Tibetan Buddhist 
practice of dream Yoga, and the ancient Indian Hindu practice of Yoga nidra. The cultivation of such awareness was common practice among early Buddhists.
17th century
Philosopher and physician Sir Thomas Browne (1605–1682)  was fascinated by dreams and described his own ability to lucid dream in his Religio Medici,
 stating: '...yet in one dream I can compose a whole Comedy, behold the 
action, apprehend the jests and laugh my self-awake at the conceits 
thereof'.
Samuel Pepys
 in his diary entry for 15 August 1665 records a dream, stating: "I had 
my Lady Castlemayne in my arms and was admitted to use all the dalliance
 I desired with her, and then dreamt that this could not be awake, but 
that it was only a dream".
19th century
In 1867, the French sinologist Marie-Jean-Léon, Marquis d'Hervey de Saint Denys  anonymously published Les Rêves et Les Moyens de Les Diriger; Observations Pratiques
 ('Dreams and the ways to direct them; practical observations'), in 
which he describes his own experiences of lucid dreaming, and proposes 
that it is possible for anyone to learn to dream consciously.
20th century
Frederik van Eeden and Marquis d'Hervey de Saint Denys, pioneers of lucid dreaming.
In 1913, Dutch psychiatrist and writer Frederik (Willem) van Eeden (1860–1932) coined the term 'lucid dream' in an article entitled "A Study of Dreams".
Some have suggested that the term is a misnomer because van Eeden
 was referring to a phenomenon more specific than a lucid dream. Van Eeden intended the term lucid to denote "having insight", as in the phrase a lucid interval applied to someone in temporary remission from a psychosis, rather than as a reference to the perceptual quality of the experience, which may or may not be clear and vivid.
Scientific research
In 1968, Celia Green
 analyzed the main characteristics of such dreams, reviewing previously 
published literature on the subject and incorporating new data from 
participants of her own. She concluded that lucid dreams were a category
 of experience quite distinct from ordinary dreams and said they were 
associated with rapid eye movement sleep (REM sleep). Green was also the first to link lucid dreams to the phenomenon of false awakenings.
Lucid dreaming was subsequently researched by asking dreamers to 
perform pre-determined physical responses while experiencing a dream, 
including eye movement signals.
In 1980, Stephen LaBerge at Stanford University developed such techniques as part of his doctoral dissertation.  In 1985, LaBerge performed a pilot study that showed that time perception
 while counting during a lucid dream is about the same as during waking 
life. Lucid dreamers counted out ten seconds while dreaming, signaling 
the start and the end of the count with a pre-arranged eye signal 
measured with electrooculogram recording. LaBerge's results were confirmed by German researchers D. Erlacher and M. Schredl in 2004.
In a further study by Stephen LaBerge, four subjects were 
compared either singing while dreaming or counting while dreaming. 
LaBerge found that the right hemisphere was more active during singing 
and the left hemisphere was more active during counting.
Neuroscientist J. Allan Hobson
 has hypothesized what might be occurring in the brain while lucid. The 
first step to lucid dreaming is recognizing one is dreaming. This 
recognition might occur in the dorsolateral prefrontal cortex, which is one of the few areas deactivated during REM sleep and where working memory
 occurs. Once this area is activated and the recognition of dreaming 
occurs, the dreamer must be cautious to let the dream continue but be 
conscious enough to remember that it is a dream. While maintaining this 
balance, the amygdala and parahippocampal cortex might be less intensely activated. To continue the intensity of the dream hallucinations, it is expected the pons and the parieto-occipital junction stay active.
Using electroencephalography
 (EEG) and other polysomnographical measurements, LaBerge and others 
have shown that lucid dreams begin in the Rapid Eye Movement (REM) stage
 of sleep.
 LaBerge also proposes that there are higher amounts of beta-1 frequency
 band (13–19 Hz) brain wave activity experienced by lucid dreamers, 
hence there is an increased amount of activity in the parietal lobes making lucid dreaming a conscious process.
Paul Tholey, a German Gestalt psychologist and a professor of psychology and sports science,
 originally studied dreams in order to answer the question if one dreams
 in colour or black and white. In his phenomenological research, he 
outlined an epistemological frame using critical realism.
 Tholey instructed his probands to continuously suspect waking life to 
be a dream, in order that such a habit would manifest itself during 
dreams. He called this technique for inducing lucid dreams the Reflexionstechnik reflection technique.
 Probands learned to have such lucid dreams; they observed their dream 
content and reported it soon after awakening. Tholey could examine the 
cognitive abilities of dream figures.
 Nine trained lucid dreamers were directed to set other dream figures 
arithmetic and verbal tasks during lucid dreaming. Dream figures who 
agreed to perform the tasks proved more successful in verbal than in 
arithmetic tasks. Tholey discussed his scientific results with Stephen 
LaBerge, who has a similar approach.
Alternative theories
Other researchers suggest that lucid dreaming is not a state of sleep, but of brief wakefulness, or "micro-awakening". Experiments by Stephen LaBerge used "perception of the outside world" as a criterion for wakefulness while studying lucid dreamers, and their sleep state was corroborated with physiological measurements.
 LaBerge's subjects experienced their lucid dream while in a state of 
REM, which critics felt may mean that the subjects are fully awake. J 
Allen Hobson responded that lucid dreaming must be a state of both 
waking and dreaming.
Philosopher Norman Malcolm
 has argued against the possibility of checking the accuracy of dream 
reports, pointing out that "the only criterion of the truth of a 
statement that someone has had a certain dream is, essentially, his 
saying so."
Definition
Paul Tholey laid the epistemological
 basis for the research of lucid dreams, proposing seven different 
conditions of clarity that a dream must fulfill in order to be defined 
as a lucid dream:
- Awareness of the dream state (orientation)
 - Awareness of the capacity to make decisions
 - Awareness of memory functions
 - Awareness of self
 - Awareness of the dream environment
 - Awareness of the meaning of the dream
 - Awareness of concentration and focus (the subjective clarity of that state).
 
Later, in 1992, a study by Deirdre Barrett examined whether lucid dreams contained four "corollaries" of lucidity:
- The dreamer is aware that they are dreaming
 - Objects disappear after waking
 - Physical laws need not apply in the dream
 - The dreamer has a clear memory of the waking world
 
Barrett found less than a quarter of lucidity accounts exhibited all four.
Subsequently, Stephen LaBerge
 studied the prevalence of being able to control the dream scenario 
among lucid dreams, and found that while dream control and dream 
awareness are correlated, neither requires the other. LaBerge found 
dreams that exhibit one clearly without the capacity for the other; 
also, in some dreams where the dreamer is lucid and aware they could 
exercise control, they choose simply to observe.
Prevalence and frequency
In 2016, a meta-analytic study by David Saunders and colleagues on 34 lucid dreaming studies, taken from a period of 50 years, 
demonstrated that 55% of a pooled sample of 24,282 people claimed to 
have experienced lucid dreams at least once or more in their lifetime. 
Furthermore, for those that stated they did experience lucid dreams, 
approximately 23% reported to experience them on a regular basis, as 
often as once a month or more.
Suggested applications
Treating nightmares
It has been suggested that sufferers of nightmares
 could benefit from the ability to be aware they are indeed dreaming. A 
pilot study was performed in 2006 that showed that lucid dreaming 
therapy treatment was successful in reducing nightmare frequency. This 
treatment consisted of exposure to the idea, mastery of the technique, 
and lucidity exercises. It was not clear what aspects of the treatment 
were responsible for the success of overcoming nightmares, though the 
treatment as a whole was said to be successful.
Australian psychologist Milan Colic has explored the application of principles from narrative therapy
 to clients' lucid dreams, to reduce the impact not only of nightmares 
during sleep but also depression, self-mutilation, and other problems in
 waking life. Colic found that therapeutic conversations could reduce 
the distressing content of dreams, while understandings about life—and 
even characters—from lucid dreams could be applied to their lives with 
marked therapeutic benefits.
Psychotherapists have applied lucid dreaming as a part of 
therapy. Studies have shown that, by inducing a lucid dream, recurrent 
nightmares can be alleviated. It is unclear whether this alleviation is 
due to lucidity or the ability to alter the dream itself. A 2006 study 
performed by Victor Spoormaker and Van den Bout evaluated the validity 
of lucid dreaming treatment (LDT) in chronic nightmare sufferers.
 LDT is composed of exposure, mastery and lucidity exercises. Results of
 lucid dreaming treatment revealed that the nightmare frequency of the 
treatment groups had decreased. In another study, Spoormaker, Van den 
Bout, and Meijer (2003) investigated lucid dreaming treatment for 
nightmares by testing eight subjects who received a one-hour individual 
session, which consisted of lucid dreaming exercises. The results of the study revealed that the nightmare frequency had decreased and the sleep quality had slightly increased. 
Holzinger, Klösch, and Saletu managed a psychotherapy study under
 the working name of ‘Cognition during dreaming – a therapeutic 
intervention in nightmares’, which included 40 subjects, men and women, 
18–50 years old, whose life quality was significantly altered by 
nightmares.
 The test subjects were administered Gestalt group therapy and 24 of 
them were also taught to enter the state of lucid dreaming by Holzinger.
 This was purposefully taught in order to change the course of their 
nightmares. The subjects then reported the diminishment of their 
nightmare prevalence from 2–3 times a week to 2–3 times per month.
Creativity
In her book The Committee of Sleep, Deirdre Barrett
 describes how some experienced lucid dreamers have learned to remember 
specific practical goals such as artists looking for inspiration seeking
 a show of their own work once they become lucid or computer programmers
 looking for a screen with their desired code. However, most of these 
dreamers had many experiences of failing to recall waking objectives 
before gaining this level of control.
Exploring the World of Lucid Dreaming by Stephen LaBerge and Howard Rheingold
 (1990) discusses creativity within dreams and lucid dreams, including 
testimonials from a number of people who claim they have used the 
practice of lucid dreaming to help them solve a number of creative 
issues, from an aspiring parent thinking of potential baby names to a 
surgeon practicing surgical techniques. The authors discuss how 
creativity in dreams could stem from "conscious access to the contents 
of our unconscious minds"; access to "tacit knowledge" - the things we 
know but can't explain, or things we know but are unaware that we know.
In popular culture
Risks
Though 
lucid dreaming can be beneficial to a number of aspects of life, some 
risks have been suggested. People who have never had the experience of 
lucid dreaming may not understand what is happening when they first 
experience a lucid dream. The person who lucid dreams could begin to 
feel isolated from others due to feeling different. It could become more
 difficult over time to wake up from a lucid dream. Someone struggling 
with certain mental illnesses could find it hard to be able to tell the 
difference between reality and the actual dream.
Long term risks with lucid dreaming have not been extensively studied.