The prevalence of dental caries is influenced by various factors, including diet, oral hygiene practices, access to oral health care, and socioeconomic status ^[1][2][20,21]. Dental caries affect 60% to 90% of school-aged children ^[3][22]. The WHO recommends implementing community-based programs to prevent dental caries and promote oral health, such as fluoridation of water supplies and sugar reduction campaigns ^[4][23].

The oral microbiota plays a crucial role in dental health and demineralization. The oral microbiota helps maintain the health of the teeth and gums by creating a protective biofilm on the tooth surfaces, which helps prevent the colonization of harmful bacteria. However, if the oral microbiota is disrupted or imbalanced, harmful bacteria can proliferate and cause dental problems such as tooth decay and gum disease ^[5][24]. Several bacteria species are associated with dental caries. The most significant cariogenic bacteria include Streptococcus mutans, Streptococcus sobrinus, Lactobacillus acidophilus, and Actinomyces species ^[6][25]. These bacteria species produce acids that dissolve the minerals in the tooth’s enamel, leading to demineralization and, ultimately, cavities ^[7][26]. After consuming sugary or starchy foods, bacteria ferment the carbohydrates, thereby producing lactic acid, which lowers the pH in the oral cavity and contributes to tooth decay ^[8][27]. This acidic environment can weaken the tooth’s enamel by demineralization and allow the bacteria to penetrate the tooth, leading to dental caries ^[9][28]. S. mutans and S. sobrinus are considered the most important cariogenic bacteria due to their ability to produce large amounts of acid and form plaque on the tooth surface ^[10][29]. L. acidophilus and Actinomyces species are also cariogenic bacteria that can contribute to the development of dental caries ^[6][11][5,25]. L. acidophilus produces lactic acid from carbohydrates and can thrive in acidic environments ^[12][30]. Actinomyces species are known to produce enzymes that can break down proteins, which can contribute to the formation of cavities ^[13][31]. Preventing dental caries involves reducing the number of cariogenic bacteria in the oral cavity through good oral hygiene practices and a healthy diet ^[14][15][32,33]. Consuming probiotics can help promote healthy oral microbiota and prevent the proliferation of harmful bacteria ^[5][24].

Caries’ development is associated, among those mentioned, with biological, physical, environmental, and behavioral factors, such as poor saliva flow, insufficient exposure to fluoride, and low socioeconomic status ^[16][17][19,34]. Risk factors for the development of caries are sociodemographic, factors related to diet, factors related to oral hygiene habits, factors related to oral bacterial flora, and other factors such as genetic background and smoking ^[2][21]. Thus, poor oral hygiene increases dental caries risk by not regularly removing plaque from the teeth through brushing and flossing ^[18][35]. Consuming foods and drinks high in sugar or acid can contribute to tooth decay ^[4][23]. Moreover, a diet lacking essential nutrients such as calcium, vitamin D, and phosphorus can weaken tooth enamel and increase the risk of dental caries ^[19][36]. A lack of saliva in the mouth can increase the risk of dental caries because saliva helps to neutralize acid and wash away food particles and bacteria. Certain medications, medical conditions, and lifestyle factors such as smoking can cause dry mouth or xerostomia ^[20][37]. Fluoride is a mineral that helps strengthen tooth enamel and protect against tooth decay. Insufficient fluoride exposure, either through drinking water, toothpaste, or other sources, can increase the risk of dental caries ^[21][38]. Children and older adults are at higher risk of dental caries. Children’s teeth are still developing and may not fully form enamel, while older adults may have more exposed root surfaces due to gum recession ^[22][23][39,40]. Some individuals may be more genetically predisposed to developing dental caries due to differences in the structure and composition of their teeth ^[24][41].

The commonly used method for measuring dental caries’ occurrence in populations is the DMFT index. DMFT stands for decayed, missing, and filled teeth. The index is calculated by counting the number of teeth in a person’s mouth that have decayed, are missing due to caries, or have been filled due to caries ^[25][47]. A higher DMFT score indicates more significant caries in the individual ^[26][48]. The DMFT index can be used to assess the prevalence and severity of dental caries in a population, as well as to monitor changes in caries’ occurrence over time. It is also helpful for evaluating the effectiveness of oral health programs and interventions to reduce caries’ prevalence ^[27][49].

In addition to the DMFT index, caries biomarkers are reduced saliva flow and an increased number of S. mutans. A decreased quantity or quality of saliva can increase the risk of dental caries ^[28][50]. Saliva is essential for regulating the oral microbiome and maintaining oral health, especially in preventing oral diseases and controlling oral infections ^[29][51]. Normal saliva secretion and saliva flow are directly related to oral health. Decreased saliva flow is an important indicator of oral health, and disorders of the salivary glands are the main factor affecting the decrease in saliva flow ^[29][51]. Saliva has the most significant influence on caries’ progression ^[30][52]. A reduction in saliva at night favors the progression of caries ^[30][52]. Moreover, saliva’s buffering capacity depends on saliva’s production at rest ^[31][53], and it is considered that the risk of caries increases in children who sleep less due to a decrease in saliva production ^[31][53]. Interestingly, the higher the DMFT index, the lower the saliva flow ^[32][54].

Saliva flow and production have been shown to exhibit a circadian pattern, with peak flow rates occurring during the day and lower flow rates at night ^[33][34][55,56]. The suprachiasmatic nucleus regulates the circadian rhythm of salivary flow and production, the body’s central circadian clock in the hypothalamus ^[33][55]. Saliva flow, salivary protein concentration, and electrolytes have a circadian pattern and are essential in maintaining and protecting oral health ^[30][52]. Saliva flow is weak in the morning, increases in the afternoon, and then decreases ^[35][57]. The circadian rhythm of the salivary glands plays a vital role in controlling food intake and the immune system because it affects the flow of saliva and the ionic composition ^[29][36][51,58]. Changes in saliva flow and composition can affect oral health, as saliva helps to neutralize acids produced by oral bacteria, remineralize tooth enamel, and lubricate the mouth ^[37][59]. Saliva is important for maintaining tooth remineralization, and people with xerostomia have an increased number of S. mutans and other acidogenic species, which favors caries’ development ^[30][52]. In addition, sleep disturbances can lead to circadian rhythm disturbances, affecting saliva production, which partially explains the connection between insufficient sleep and caries’ development ^[28][38][50,60]. Furthermore, disruption of the circadian rhythm, such as chronic sleep disturbance, may affect saliva flow and composition, contributing to oral health problems, including dental caries ^[32][54].

There is a connection between the amount of sleep and dental caries. Studies have shown that poor sleep or poor sleep quality can increase the risk of developing dental caries ^[8][26][27,48]. Moreover, some studies have suggested that poor sleep quality and insufficient sleep may be associated with a higher risk of dental caries, as reflected by the DMFT index ^[7][39][40][26,63,64]. One reason for this connection is that insufficient sleep can lead to changes in the body’s immune system, making it less effective at fighting off bacteria that cause dental caries ^[41][65]. Additionally, during sleep, saliva production is reduced, which can cause a decrease in the neutralization of acids produced by oral bacteria that can lead to enamel erosion and tooth decay ^[42][66].

Insufficient sleep has been associated with an increased risk of dental caries in adolescents ^[40][64]. Several studies have investigated the relationship between sleep duration or sleep quality and dental caries in adolescents, with some suggesting that insufficient sleep may be a risk factor for caries ^{[7][39][40][41]}[26,63,64,65]. Thus, Sardana et al. reported that irregular or late bedtimes and fewer hours of sleep were independent risk factors for caries in early childhood ^[7][26]. Ogawa et al. reached a similar conclusion in their study, reporting that sleep duration was independently associated with caries in early childhood ^[43][68]. Duration and quality of sleep were significantly related to symptoms of dental disease in a study by Choi et al. ^[41][65]. Moro et al. took a slightly different approach and examined sleep problems in school children due to dental issues and came to similar conclusions—sleep problems in school children are associated with untreated dental caries ^[40][64]. Although these studies were conducted on different continents, they all came to one solid conclusion—sleep is a risk factor for the development of dental caries. The mechanisms by which insufficient sleep may contribute to adolescents’ caries’ development are not fully understood. Still, it has been suggested that changes in saliva flow and composition, as well as alterations in dietary habits and oral hygiene practices, may play a role ^[11][22][44][2,5,39]. Insufficient sleep can affect the production and composition of saliva, reducing its ability to neutralize acids produced by oral bacteria. In addition, insufficient sleep has been linked to poor dietary habits, such as increased consumption of sugary and acidic foods and drinks, which can contribute to the development of dental caries ^[22][39].

Having enough quality sleep is vital for overall health, including dental health. It can help maintain a healthy immune system and adequate saliva production, reducing the risk of developing dental caries ^[45][69]. Lack of sleep increases salivary glucose levels, decreases salivary flow, and increases the number of S. mutans, which can alter the levels of inflammatory cytokines and modify the caries formation process ^[7][43][46][11,26,68].

Moreover, children’s bedtime can have an impact on caries’ prevalence. Studies have shown that individuals who go to bed later at night risk developing dental caries more than those who go earlier ^[47][70]. In addition, later going to bed ^[48][49][9,61] and short night sleep duration are associated with an increased risk of caries ^[28][50]. In children, the sleeping routine is also essential, and those children who had an irregular bedtime had a higher prevalence of caries ^[28][50]. One possible explanation for this association is that staying up late at night can lead to irregular sleep patterns, disrupting the circadian rhythm and decreasing saliva production, making it harder for the body to neutralize the acids produced by oral bacteria ^[50][71].

Additionally, individuals who stay up late may be more likely to consume sugary or acidic foods and drinks, which can also contribute to tooth decay ^[48][9]. In addition, due to going to sleep later, hormones related to appetite are stimulated, which leads to increased food intake and snacking—night eating ^[8][48][9,27]. Furthermore, staying up late can lead to poor oral hygiene practices, such as skipping brushing or flossing before bed ^[26][48]. Neglecting these practices can allow oral bacteria to thrive, increasing the risk of developing dental caries. Children who stay awake longer have a higher risk of developing caries and brush their teeth less often. Shorter sleep duration is associated with increased S. mutans in saliva ^[8][48][9,27]. Thus, the research showed that children who went to sleep irregularly had an increased risk of caries’ development ^[7][26]. Children who went to sleep after 11 p.m. also had an increased risk of tooth decay ^[7][26]. The prevalence of caries was lower in children who went to bed before 9 p.m. ^[47][70].

Sleep habits can have a significant impact on a child’s oral health. Poor sleep habits, such as fragmented sleep ^[49][61] and unhealthy sleep habits of parents, have been associated with a higher prevalence of dental caries in children ^[39][63]. Allowing children to eat sugary products before bedtime has also been associated with higher caries’ prevalence ^[26][48]. In contrast, maintaining a healthy bedtime routine may lead to healthier teeth in children ^[26][48]. In addition, it is important to note that dental caries can negatively affect family life and lead to sleep disturbances in children ^[3][22]. Therefore, establishing a healthy lifestyle is important for preventing dental caries in children ^[39][63], and children who slept longer had a lower prevalence of caries ^[50][71]. Maintaining a regular bedtime and good sleep hygiene practices can help reduce the risk of developing dental caries ^[26][48].

Sleep duration has been linked to caries’ development, with short and long sleep durations associated with an increased risk of dental caries. Short sleep duration, typically defined as less than 7 h per night, has been associated with an increased risk of dental caries in children and adults ^[7][39][26,63]. Long sleep duration, typically defined as more than 9 h per night, has also been associated with an increased risk of dental caries, particularly in children ^[40][64]. Research has shown that children who slept less than 8 h during the night had an increased risk of caries compared to children who slept more than 11 h ^[7][31][26,53]. Insufficient sleep in children is associated with an increased risk of tooth decay. The shorter the sleep duration, the greater the risk ^[28][31][50,53]. A possible explanation is that with fewer hours of sleep, there is an increase in the sympathetic nervous system, which causes a weakening of the immune system.

The exact mechanisms by which sleep duration influences caries’ development are poorly understood. Still, changes in saliva flow, oral microbiota composition, and dietary habits may play a role in caries’ development. For example, short sleep duration has been associated with decreased saliva flow, which can reduce the ability of saliva to neutralize acids produced by oral bacteria and remineralize tooth enamel ^[37][51][59,73]. Long sleep duration may also disrupt normal saliva flow patterns, leading to changes in oral microbiota composition and an increased risk of caries ^[52][74].

Chronotype refers to the innate biological predisposition of an individual to prefer specific sleep–wake patterns ^[53][83]. There are three categories of chronotypes: morning, intermediate, and evening. Morning types prefer to wake up early and feel most alert during the early hours of the day. Evening types prefer to stay up late at night and feel most alert later in the day. Intermediate types fall somewhere between and may not strongly prefer either morning or evening ^[54][84]. Genetics largely determines chronotype and can be influenced by age, lifestyle, and environmental factors. Understanding an individual’s chronotype can help optimize their sleep–wake cycle. It can inform the best times to sleep, wake up, and engage in daily activities ^[55][15]. Individuals with an evening chronotype may be more likely to have poor sleep quality and experience sleep disorders such as insomnia and sleep apnea ^[56][57][58][85,86,87]. Poor sleep quality and sleep disorders can negatively affect overall health, including an increased risk of obesity, diabetes, cardiovascular disease, and depression ^[16][19]. Morning-type adolescents had lower DMFT scores than evening types ^[16][19]. One possible explanation for this association is that individuals with an evening chronotype may have irregular sleep patterns, leading to decreased saliva production and poor oral hygiene practices, which can increase the risk of dental caries ^[35][57].

Individuals with an evening chronotype may have a delay in the timing of saliva production compared to those with a morning chronotype ^[29][56][51,85]. This delay can decrease saliva production in the morning, increasing the risk of dental caries. Conversely, morning chronotype is a protective factor for caries ^[59][89]. Furthermore, studies have shown that individuals with an evening chronotype may have a higher prevalence of sleep disorders such as sleep apnea ^[60][90], which can cause dry mouth and reduce saliva production. This reduction in saliva can increase the risk of developing dental caries ^[56][85]. However, it is important to note that while chronotype can influence saliva production, many other factors can also affect it.

Moreover, oral health in people with an evening chronotype is worse due to night eating syndrome. Frequent night eating predicts poor oral health significantly ^[61][62][88,91]. Children with daily life habits related to the evening chronotype have a higher prevalence of dental caries ^[35][57]. Eating late at night when saliva secretion is reduced can lead to a higher prevalence of caries ^[35][57]. Some studies have shown that evening chronotypes have a higher risk of periodontal disease and caries precisely because of night eating and insomnia, which is much less represented in morning chronotypes ^[61][88]. As the evening chronotype is associated with poor habits (skipping breakfast, brushing teeth less often, eating at night), evening-type adolescents had worse oral health and a higher risk of caries than morning types ^[59][61][88,89]. However, the circadian rhythm changes throughout life, and morning fatigue is more common during puberty than later in life ^[59][89]. That is why parents’ healthy habits—regular bedtime, breakfast, reduced intake of sweets and juices, and oral hygiene—are important factors in preventing caries in children ^[43][68]. Individuals with evening chronotype may have shorter sleep duration, delayed sleep onset, and irregular sleep patterns ^[56][85], which could affect saliva production, reduce immune function, and alter oral microbiota composition, all of which are factors that can contribute to the development of dental caries ^[6][63][25,92]. 

The circadian rhythm regulates the sleep–wake cycle, and poor sleep can disrupt the circadian rhythm and vice versa ^[64][94]. These two processes determine most aspects of sleep and related variables such as sleepiness and wakefulness ^[65][95]. Disrupting the circadian rhythm can also lead to poor sleep quality, contributing to dental caries’ development ^[39][43][63,68]. As mentioned, the circadian rhythm affects saliva production, which is essential in maintaining oral health by neutralizing acids produced by oral bacteria that can cause tooth decay ^[33][55]. Disrupting the circadian rhythm through poor sleep can lead to inadequate saliva production, increasing the risk of developing dental caries ^[28][50]. A healthy circadian rhythm will benefit sleep, and good sleep will strengthen the creation of the rhythm. Contrarily, disturbed sleep can result in a less functional circadian rhythm, reducing sleep quality ^[65][95]. Conversely, dental caries can also disrupt the circadian rhythm. In addition, pain from tooth decay can interfere with sleep and lead to poor sleep quality, further disrupting the circadian rhythm ^[29][42][51,66]. This disruption can then cause a decrease in saliva production and immune function, making it harder for the body to fight off bacteria that cause tooth decay.

The circadian rhythm is crucial in maintaining sleep quality and oral health. Disruptions to this rhythm from poor sleep or dental caries can lead to a vicious cycle of poor sleep and oral health outcomes. Therefore, it is essential to maintain good sleep habits and seek treatment for dental caries promptly to minimize the risk of disrupting the circadian rhythm and its associated adverse effects.