Hypnopedia in Pediatric Dentistry: A Concept That Warrants Exploration

Every day, numerous memories are stored in a person's brain. At the end of a day, people can remember certain individuals, events, places, thoughts, and experiences. As each memory enters one's brain, it leaves a trace in the brain. Although much learning takes place each day, most of what is learned is forgotten. Some is retained, but little endures forever. This may elicit the question as to why some memories endure indefinitely. One answer is "practice." Memories that are retained are those that are frequently rehearsed. It has always been thought that people must be awake when learning a new skill, remembering new information, or rehearsing newly acquired knowledge.¹ Now, however, the concept of sleep learning may be considered.

Sleep is essential for all human beings, and it naturally aids learning. Sleep deprivation has repeatedly been proven to lead to deterioration of one's general health and well-being. Although learning happens when people are awake, it does not occur in a single moment. When learning something new, it has to be rehearsed and memorized frequently so that it stays in one's learning scheme for a long time.² Rehearsal can occur even during sleep, when one doesn't even realize it is happening. The effect of sleep on general and mental health is well known, yet if the importance of sleep on learning is also considered, it is clear that sleep has tremendous significance in people's lives.³ When specific memories stored in the brain become active during sleep, this helps in establishing memories to be stored for longer periods.⁴ While various studies have implicated sleep in memory stabilization, the experiment on targeted memory reactivation (TMR) has provided the most direct evidence of sleep learning.^5-7

The experiment stated that an odorant presented during sleep increased learning of spatial locations. TMR can also be accomplished by presenting low-intensity auditory stimulus during slow-wave sleep to promote memory rehearsal without the subject awaking. TMR allows humans to modify certain memories. Many experiments have been done to show that a sound stimulus combined with verbal or non-verbal learning material and the control of the gender of the voice of the sound stimulus decreases oblivion and enhances accuracy in recall.^8-10 Also, TMR enhances activity in the right hippocampal cortex, and the activity that is recorded during sound generation in the thalamus, temporal lobe, and cerebellum is all positively related to memory accuracy. The brain gets activated in different ways during familiar and unfamiliar acoustic stimuli. This technique greatly affects memory, even in slow oscillations of the brainwaves.¹¹

Before discussing the topic of sleep learning, it is important to know how new learning takes place. Initially, new information acquired must be understood in the context of current knowledge. At the same time, acquiring new knowledge should not have a negative impact on already-existing knowledge. New information is retained and assimilated through the process of consolidation. To learn newer facts and events (represented as declarative memories), consolidation depends on interactions among sets of networks in the cerebral cortex and hippocampus.¹² Through these interactions, consolidation can entail changes in how information is represented within cortical networks. Other types of memories, including learned skills, consolidate through other areas of the brain.¹³ Retrieving knowledge that has been recently acquired depends on activation of associations between previous knowledge and the new knowledge.¹⁴ The stored memory gets strengthened when new knowledge is integrated with already-existing knowledge. Sleep can play an important role in this consolidation process.

When people take new information into account, they learn to adjust to environmental demands so they can achieve their goals. That is, a person's brain always tries to adapt and show optimal behavior according to the surrounding environment and, thus, fulfill the goals the person has set up for themself. For example, people who need to achieve good grades on an academic test have to train themselves accordingly.¹⁵ On any given day, people are exposed to both important (eg, meeting new people, learning new skills) and unimportant (eg, advertisements for unwanted products) information. Later that same night, they could engage in suboptimal sleep-dependent consolidation if they rehearse the information they would prefer to forget instead of the information they wish to retain. Learning during sleep is possible by the synaptic-homeostasis mechanism. While the brain is awake, synapses are heavily charged. During sleep, the synapses are unloaded by slow brainwaves and get stabilized. In this phase, memory consolidation is achieved, especially after learning a task.¹⁶ The memory process includes information coding, consolidation, interacting with information, information recall, and confirmation. Memory consolidation is favored by sleep in memory, whether before or after the learning process. Newly learned memory traces have been reported in an important timeframe after learning when they are still fragile and susceptible to disruptive stimulus interference. Research indicates that periods of sleep occurring in this window can facilitate the process of memory consolidation.¹⁷ Memory consolidation entails both quantitative and qualitative changes. Two hypotheses have been put forward: one states declarative memory is increased in NREM (non-rapid eye movement) sleep, whereas procedural memory is better in REM (rapid eye movement) sleep. In another hypothesis, hippocampus and cortex are in interaction in slow-wave sleep, and then REM phase positively contributes in memory consolidation.¹⁸

Sleep is a state in which minimal mental and physical activity is exerted. Almost all of the senses reject responding to the stimuli of the environment. The response to any vocal stimulus is scattered, and present memories are disrupted. Despite this, sleeping and memorizing processes are strongly neurally interrelated. Thus, humans learn things even while asleep.

There are two kinds of sleep: NREM and REM.¹⁹ In NREM sleep, brainwaves appear slowly on an electroencephalogram (EEG), which measures electrical activity in the brain; spindles of sleep in the range of 12 to 16 hertz as well as K-complexes are seen. Brainwaves tend to move slower in sleep and are important for processing information and making memories. There are four phases in NREM sleep as recorded via EEG. They are differentiated by the speed of the brainwaves and the phases repeat themselves every 90 to 100 minutes.

In REM sleep, the brain shows activity and the EEG is somewhat disorganized. REM sleep occurs about 1.5 hours after a person goes to sleep. The initial 5 to 10 minutes of REM sleep is the phase of dreaming. The brain activity is markedly increased in this phase. The ability to learn while sleeping is made possible by the synaptic-homeostasis mechanism. When the brain is not sleeping, synapses are highly activated. While in sleep the synapses unload via the slow brainwaves and become stable. This is the stage where memory gets consolidated, especially after the mind has learned new tasks.²⁰

Cognition is also influenced by physiological procedures while sleeping, such as inactivity, equilibrium, and biological rhythms. Hindrances are affected by biological rhythms, and selected-optical awareness is affected by inactivity. In addition, constant-optical attention, work-memory, timely response, frame of mind, alertness, and inspiration are affected by equilibrium and biological rhythms.²¹

"Hypnopedia," or "sleep learning," simply stated is the act of learning new things when sleeping. It is said to be a technique based on neural-linguistic programs that help humans reframe the mind for multiple varied uses.²² This is possible because when people sleep, their unconscious is much more receptive to the messages it receives. This concept is old and fascinating for researchers worldwide. The concept that humans may be able to learn while in sleep, known as hypnopedia, has a varied and peculiar historical background. One of the first references to this concept was from 1927 when an American, A.B. Saliger, invented the Psycho-phone. He considered the instrument an "automatic suggestion machine." The Psycho-phone was a phonograph with a clock installed in it. It was used to play wax cylinder records, which he made and sold.²³ Another theory suggests that hypnopedia was discovered in 1923 when the US Marines learned Morse codes in sleep. In his book Brave New World, Aldous Huxley gave the example of a Polish boy who recited a broadcast in English, which he heard in his sleep, indicating Huxley knew of this new method of learning. This fictional concept spawned much research through the 1950s on sleep learning.²⁴

One of the first experiments pertinent to pediatric dentists involved the cessation of nail-biting habits in children via hypnopedia. Performed by psychologist Lawrence LeShan, the research entailed repeatedly playing a record of a single sentence 50 times that said, "My fingernails taste terribly bitter." It was played to a group of 20 children six times per night for 54 nights. After 54 nights, 40% of the children had stopped their nail-biting habit.²⁵

Scientists have neither approved nor debunked the concept of hypnopedia. The idea has been revived with new vigor recently as studies have suggested hypnopedia can be a beneficial tool for learning.²⁶Memory is a basic cognitive function of learning, which is why much research has been done on the relationship between sleep and memory. Increased cognitive properties, such as analytical and innovative, may evolve during a night's sleep. Sleep may intensify one's learning ability, thus hypnopedia may be an additional means of boosting cognitive abilities and learning.

This concept can be used in pediatric dentistry in managing children with behavioral problems. When children behave inappropriately in a dental clinic and do not listen to the dentist's or other staff members' commands or instructions, the use of hypnopedia may be considered to attempt to change the scenario. For 21 continuous nights prior to the dental appointment, the parent or guardian might say messages to an aggressive child while he or she is sleeping, such as "You are kind and loving to others," or "You feel very happy while at the dentist." The idea is that cell regeneration takes place every 21 days, so the child's new cells will be uploaded with new information regarding the clinic environment. Another advantage that can be derived from hypnopedia is increased self-esteem of the child, which can aid in overcoming shyness. These benefits can, in turn, help pediatric dentists when managing the child in the office, especially fearful or shy children. This strategy can also reinforce their capabilities and enhance their evolution during their sleep time.²⁷

Sleep is a principal part of motor memory amalgamation and education, providing a critical tool to increase coaching of new habits of learning and a complete overhaul of this learning. Once the initial skill of learning is acquired, memory stabilization is mostly due to NREM sleep rather than a complete night's sleep or nap. TMR is utilized to increase this important procedure of memory stabilization, which pairs an external signal with performance of a function at the time of a newly acquired motor skill, which is followed by repetition of the same signal in sleep. Implementation of TMR while sleeping provides guidance to enhanced practical association in job-oriented brain networks and enhanced behavior in healthy young people. Memory reactivation during sleep can shape new memories into a long-term form. TMR is thought to manipulate reactivation on demand. In this method, sensory cues such as odors or sounds are paired with information during wake, and then re-presented during subsequent sleep. This can lead to a behavioral memory benefit, a result which seems to suggest that the manipulation may increase memory replay.²⁸

Hypnopedia might be able to allow information to penetrate the brain and could increase cognition. More studies are necessary to confirm the benefits of hypnopedia, although the use of TMR or other approaches, in different areas and groups, can lead to better learning. Such outcomes will help in organizing learning environments and sleep routines accordingly. Suggestions during sleep might regulate behaviors, emotions, and analytic abilities. Overall, hypnopedia might enhance useful behaviors and/or emotional health. This is a novel study area. More knowledge needs to be extracted from studies on brain prowess and analytic capabilities, even during sleeping, regarding how to better achieve a higher level of cognition and healthy state of mind.

Nearly one-third of people's lives is spent sleeping, which although essential, seems to be a waste of a lot of time that could be utilized doing important works. It would be very interesting if people could maximize learning while sleeping. This would be quite valuable, for example, for students who need to do last-minute cramming before exams!

Huxley's Brave New World introduced a phenomenon that has been neither accepted nor outrightly rejected by the scientific world. Early on, hypnopedia was thought to be fictional, but now with scientific literature supporting this decades-old concept, it is being studied as a behavior modification tool. Researchers have studied it under various ideas and themes. Huxley's work provided the scientific community the hope of making up for lost time to learn whether or not it is possible to learn when asleep. The level of evidence provided by studies on hypnopedia is low, but researchers remain hopeful, especially when the concept is applied in simpler forms of learning.²⁹

Hypnopedia was rejected in the 1960s after an experiment by Emmons and Simon, in which 10 one-syllable nouns were repeated to nine subjects in their sleep. The next day the subjects were asked to select the nouns they heard at night while sleeping from a list of 50 words. They selected the words by chance and did not fare better than the control group who had not heard the words. Although the experiment had flaws, the scientific community was convinced that hypnopedia was not possible and interest in this subject waned. The concept, however, was further explored in Russia, where some experiments produced excellent results.³⁰ The works of Hoskovec in 1961 and Rubin in 1968 and 1971 showed that people can learn new things-although of a simple nature-when asleep. One reason for the difference between the Russian studies and Western studies could be that the Russians tested recognition rather than recall. Despite the positive results by Russian scientists, the results failed to generate any meaningful enthusiasm in Western scientists, and the topic of hypnopedia went into obscurity.³¹

While hypnopedia has been the subject of pop culture and science fiction books and movies, recent neurological research on memory has brought the idea back to the forefront.³² Sleeping state is known to be an inactive state of mind and body, and it is closely related to memory consolidation and enhancement of cognition. Because memory and learning are interconnected, a new field of research has evolved on sleep learning or hypnopedia.

Hypnopedia is a concept that has fascinated scientists since the concept was first put forward. Although it has been debunked by many, it continues to hold a place in sleep medicine and pediatrics. This concept has not been explored yet in pediatric dentistry but might be useful for instilling a positive attitude in children regarding their dental visits and oral health. Children who show uncooperative behavior in dental clinics may be made to accept the clinic environment through the use of hypnopedia. This can work by focusing on the behaviors dentists want to encourage.

Parents/guardians can sleep-talk to a child to raise his or her self-confidence. Here, the choice of words/sentences is important. First, it must be decided what choice of messaging will be delivered to the child. The spoken message to the child must be specific and be beneficial in the clinical environment. Messages must be short, positive, and limited in number. They should be affirmative. A pediatric dentist might consider training the parent of an uncooperative child on use of this concept for the benefit of the child, the dentist, and ultimately the child's oral health. This concept also can be applied to shun or deter deleterious oral habits. The concept of hypnopedia should be further explored for application in pediatric dentistry, as it could become an important method of behavior management.

Sonu Acharya, BDS, MDS
Professor, Pediatric and Preventive Dentistry, Institute of Dental Sciences, S'O'A (deemed to be) University, Bhubaneswar, Odisha, India; Private Practice, Bhubaneswar, Odisha, India

Sheetal Acharya, BDS, MDS
Assistant Professor, Department of Periodontics and Oral Implantology, Kalinga Institute of Dental Sciences, KIIT (deemed to be) University, Bhubaneswar, Odisha, India

Raju Biswas, BDS, MDS
Dental Surgeon, Department of Pediatric Dentistry, Dr. R. Ahmed Dental College, Kolkata, West Bengal, India

1. Antony JW, Gobel EW, O'Hare JK, et al. Cued memory reactivation during sleep influences skill learning. Nat Neurosci. 2012;15(8):1114-1116.

2. Paller KA. Sleeping in a brave new world: opportunities for improving learning and clinical outcomes through targeted memory reactivation. Curr Dir Psychol Sci. 2017;26(6):532-537.

3. Creery JD, Oudiette D, Antony JW, Paller KA. Targeted memory reactivation during sleep depends on prior learning. Sleep. 2015;38(5):755-763.

4. Diekelmann S, Born J. The memory function of sleep. Nat Rev Neurosci. 2010;11(2):114-126.

5. Marshall L, Mölle M, Hallschmid M, Born J. Transcranial direct current stimulation during sleep improves declarative memory. J Neurosci. 2004;24(44):9985-9992.

6. Kapsi S, Katsantonis S, Drigas A. Hypnopaedia or sleep learning: overview on methods and results. Res Soc Dev. 2021;10(7):e2410714721.

7. Batterink LJ, Oudiette D, Reber PJ, Paller KA. Sleep facilitates learning a new linguistic rule. Neuropsychologia. 2014;65:169-179.

8. Batterink LJ, Creery JD, Paller KA. Phase of spontaneous slow oscillations during sleep influences memory-related processing of auditory cues. J Neurosci. 2016;36(4):1401-1409.

9. Budzynski TH. Twilight learning revisited. Biofeedback. 2011;39(4):155-166.

10. Johnson BP, Scharf SM, Verceles AC, Westlake KP. Use of targeted memory reactivation enhances skill performance during a nap and enhances declarative memory during wake in healthy young adults. J Sleep Res. 2019;28(5):e12832.

11. Ngo HVV, Martinetz T, Born J, Mölle M. Auditory closed-loop stimulation of the sleep slow oscillation enhances memory. Neuron. 2013;78(3):545-553.

12. Rasch B, Büchel C, Gais S, Born J. Odor cues during slow-wave sleep prompt declarative memory consolidation. Science. 2007;315(5817):1426-1429.

13. Staresina BP, Bergmann TO, Bonnefond M, et al. Hierarchical nesting of slow oscillations, spindles and ripples in the human hippocampus during sleep. Nat Neurosci. 2015;18(11):1679-1686.

14. Rudoy JD, Voss JL, Westerberg CE, Paller KA. Strengthening individual memories by reactivating them during sleep. Science. 2009;326(5956):1079.

15. Lourenco F, Casey BJ. Adjusting behavior to changing environmental demands with development. Neurosci Biobehav Rev. 2013;37(9 Pt B):2233-2242.

16. Schouten DI, Pereira SI, Tops M, Louzada FM. State of the art on targeted memory reactivation: sleep your way to enhanced cognition. Sleep Med Rev. 2017;32:123-131.

17. Holz J, Piosczyk H, Landmann N, et al. The timing of learning before night-time sleep differentially affects declarative and procedural long-term memory consolidation in adolescents. PLoS One. 2012;7(7):e40963.

18. Westerberg CE, Florczak SM, Weintraub S, et al. Memory improvement via slow-oscillatory stimulation during sleep in older adults. Neurobiol Aging. 2015;36(9):2577-2586.

19. Westerberg CE, Mander BA, Florczak SM, et al. Concurrent impairments in sleep and memory in amnestic mild cognitive impairment. J Int Neuropsychol Soc. 2012;18(3):490-500.

20. Dudai Y. The restless engram: consolidations never end. Annu Rev Neurosci. 2012;35:227-247.

21. Teinonen T, Fellman V, Naatanen R, et al. Statistical language learning in neonates revealed by event-related brain potentials. BMC Neurosci. 2009;10:21.

22. Arzi A, Shedlesky L, Ben-Shaul M, et al. Humans can learn new information during sleep. Nat Neurosci. 2012;15(10):1460-1465.

23. Psycho-Phone. Mr. Saliger has sold more than 2500 Psycho-phones. The New Yorker. 1933;11-18.

24. Huxley A. Brave New World. New York, NY: Doubleday Doran; 1932.

25. LeShan L. The breaking of a habit by suggestion during sleep. J Abnormal Soc Psych. 1942;37(3):406-408.

26. Joensen BH, Harrington MO, Berens SC, et al. Targeted memory reactivation during sleep can induce forgetting of overlapping memories. Learn Mem. 2022;29(11):401-411.

27. Feld GB, Diekelmann S. Sleep smart-optimizing sleep for declarative learning and memory. Front Psychol. 2015;6:622.

28. Ellenbogen JM, Hulbert JC, Jiang Y, Stickgold R. The sleeping brain's influence on verbal memory: boosting resistance to interference. PLoS One. 2009;4(1):e4117.

29. Antony JW, Gobel EW, O'Hare JK, et al. Cued memory reactivation during sleep influences skill learning. Nat Neurosci. 2012;15(8):1114-1116.

30. Emmons WH, Simon CW. The non-recall of material presented during sleep. Am J Psychol. 1956;69(1):76-81.

31. Hoskovec J. Hypnopedia in the Soviet Union: a critical review of recent major experiments. Int J Clin Exp Hypn. 1966;14(4):308-315.

32. Oghenero OA. A visuo-spatial understanding of hypnopaedia. Int J Innovative Sci Res Tech. 2019;4(1):259-260.