Year : 2019  |  Volume : 61  |  Issue : 6  |  Page : 598--604

Chronotypes and its association with psychological morbidity and childhood parasomnias

Ng Syiao Wei1, Samir Kumar Praharaj2,  
1 Department of Otorhinolaryngology, Hospital Sultanah Aminah, Johor Bahru, Malaysia
2 Department of Psychiatry, Kasturba Medical College, Manipal, Manipal Academy of Higher Education, Manipal, Karnataka, India

Correspondence Address:
Dr. Samir Kumar Praharaj
Department of Psychiatry, Kasturba Medical College, Manipal, Manipal Academy of Higher Education, Manipal - 576 104, Karnataka


Background: The relationship between chronotypes and sleeping problems is not clear. The objective of the study was to identify the relative occurrence of chronotypes among college students and to explore adult psychological morbidity and childhood sleeping problems across chronotypes. Materials and Methods: One hundred and fifty undergraduate medical students were assigned into different chronotypes by Morningness–Eveningness Questionnaire and they were further assessed using Self-Reporting Questionnaire, Parasomnia Questionnaire (adapted from the Adult Sleep Disorders Questionnaire), and Pittsburgh Sleep Quality Index. Results: Intermediate chronotype was the most common, seen in 87 (58%) students, followed by evening type in 34 (22.7%). Evening types have more difficulties in making a decision, becoming exhausted more easily and feeling worthless than other chronotypes. Evening-oriented students showed a significantly higher frequency of initial insomnia and poorer overall sleep quality than the other groups. The current bedwetting was more in evening types; there was no difference in any other current and childhood parasomnias. Conclusions: Evening chronotypes had greater difficulty in decision-making, and they were more vulnerable to feel worthless. No significant association was found between childhood parasomnias and chronotypes except persistent bedwetting during adulthood in evening types.

How to cite this article:
Wei NS, Praharaj SK. Chronotypes and its association with psychological morbidity and childhood parasomnias.Indian J Psychiatry 2019;61:598-604

How to cite this URL:
Wei NS, Praharaj SK. Chronotypes and its association with psychological morbidity and childhood parasomnias. Indian J Psychiatry [serial online] 2019 [cited 2020 Aug 11 ];61:598-604
Available from:

Full Text


The chronotypes, morningness/eveningness, reflects an individual's preferred time to carry out daily activities and defines whether they are an early or late riser. It is associated with circadian variation of alertness[1] and diurnal physiological changes such as cortisol and melatonin levels and peak, amplitude, or period of core body temperature.[2],[3],[4] Studies have shown that evening chronotypes had poorer sleep quality and more stressful personality, and they are more prone to sleeping and psychological problems[5],[6] and bipolar disorder.[7],[8] Elevated risk of obsessive–compulsive symptoms is seen among persons with delayed bedtimes, while morningness has been shown to be protective against sleep-related and emotional problems.[9]

Sleeping problems are common among children. However, its prevalence is especially high among children with psychiatric disorders such as attention deficit hyperactivity disorders and comorbid anxiety/mood disorders.[10] Children who encountered any of the parasomnias had a higher risk of reporting psychotic experiences than those who had none.[11] Its persistence into adulthood is related to substance abuse,[12],[13] depressive disorders,[14] and anxiety.[15] In addition, patients who have sleep terrors and sleepwalking had been observed to have higher rates of sleep talking and nightmares as well as enuresis in childhood.[16] Children with persistent nightmares were also more likely to develop psychopathological symptoms,[11],[17],[18],[19] and their effects in adulthood are to be determined. It is important to understand the relationship between the childhood sleeping problems and adult psychiatric morbidity as indicated in the study by Fichter et al.,[14] as persistence of psychiatric symptoms even if they are subclinical, has clinical and public health implications.

Although studies on the association between chronotypes and depressive symptomology are increasingly common, no study has yet shown a direct relationship between childhood sleeping problems and chronotypes as both may be associated with psychological morbidity. This study hypothesizes the developmental process of psychological and psychiatric disorders from childhood to adulthood, which manifesting initially as childhood parasomnias and later as evening chronotypes and psychological morbidity. Thus, this will provide valuable information to understand the etiology, natural history, prevention, treatment, and prognosis of mental disorders.[14] Identification of these symptoms as prognostic factors may increase awareness of clinicians and public on childhood parasomnias and chronotypes which may be the early manifestation of the psychological illness as childhood disorders which were once thought to resolve with age had influences in later life.[20],[21] With the help of parents, early target preventive interventions such as sleep education and modifications,[22] psychological counseling can be done to prevent the emergence of projected psychological disorders or minimize persistence of such experiences.[11] The objective of the study was to characterize the chronotypes among young adults and its association with childhood parasomnias and psychological morbidity.

 Materials and Methods


This was an observational cross-sectional study involving 150 medical undergraduates (out of approximately 1250 students) of Kasturba Medical College, Manipal, Karnataka, in South India, aged between 16 and 25 years. The sample size was calculated based on the distribution of chronotypes, i.e., with a 50% prevalence rate for morning/evening chronotypes, precision 10% and a confidence interval of 95%, the sample size was 96. Exclusion criteria were frequent trans-time zone traveling and students suffering from chronic medical conditions such as diabetes, bronchial asthma, or rheumatoid arthritis. The study was approved by the institutional ethical committee and informed consents were taken prior to subjects' participation.


An individual's peak sleepiness and alertness in a day were determined using the 19-item self-administered Morningness–Eveningness Questionnaire (MEQ)[2] with total score ranges from 16 to 86. The participants were assigned according to the total score: morning (M) type (59 and above), evening (E) type (41 and below), and intermediate (I) type (42–58 score). Psychological morbidity screening was done using the 20-item Self-Reporting Questionnaire (SRQ),[23] developed by the World Health Organization. A Parasomnia Questionnaire, which has been adapted from the Sleep Disorders Questionnaire-Adult,[24] containing seven subjective questions to assess parasomnias, for example, nightmares, sleep terrors, teeth grinding, bedwetting, sleepwalking, and acting out of dream during childhood and at present, and the age of onset, the frequency of such parasomnias. The participants were asked to recall sleeping problems for the past month. For example, “Do you wake from sleep feeling very scared without an obvious reason? If yes, how frequent it is? When did they begin (the age of the first occurrence)?” The participant's sleep quality and disturbances during the preceding month was assessed using Pittsburgh Sleep Quality Index (PSQI),[25] a self-rated questionnaire containing 19 items and seven “component” scores on subjective sleep quality, sleep latency, sleep duration, habitual sleep efficiency, sleep disturbances, use of sleeping medication, and daytime dysfunction.


The students were briefed about the study, and clear instructions were given. They were reassured that the information collected will be confidential. The sociodemographic and clinical pro forma, self-rated questionnaires (MEQ, SRQ, Parasomnia questionnaire, and PSQI) together with a subject information sheet and consent form were distributed among the students fulfilling the inclusion criteria in the campus. All the forms were distributed and collected by the first author.

Statistical analysis

The data obtained from 150 participants were analyzed with the Statistical Package for Social Sciences-version 16.0 for Windows® (SPSS Inc., Chicago, IL, USA). Data normality was examined using histograms and Shapiro–Wilk test. For group comparison, Pearson's Chi-square test (or Fisher's exact test) and Kruskal–Wallis test were used for categorical and continuous data, respectively. Effect sizes were reported as Cramer's V and r for categorical and continuous data, respectively. The alpha error was kept at 5%.


Characteristics of chronotypes

Among the 150 participants, 69 (46%) were male, and the mean age was 20.69 (standard deviation [SD] 0.96). Intermediate chronotype was the most common chronotype seen in 87 (58%) students, followed by evening orientation in 34 (22.7%) and morning type in 29 (19.3%) students. The group differences among chronotypes are summarized in [Table 1]. There was no difference in age, gender, education years, number of siblings, nationality, and family type across chronotypes. Alcohol use was significantly higher in the evening (32.4%) and intermediate (20.7%) type as compared to 6.9% in morning types (P = 0.045), with small effect size (Cramer's V = 0.20). Caffeine use was significantly higher in intermediate (83.9%) and evening (79.4%) types, as compared to 62.1% among morning types (P = 0.045), with small effect size (Cramer's V = 0.203). Only five students had hypnotics uses, four were of evening types (11.8%, P = 0.023), with small effect size (Cramer's V = 0.255).{Table 1}

Psychological morbidity across chronotypes

The median SRQ score was 19 (interquartile range [IQR] 7), 17 (IQR 14), and 16 (IQR 8) among morning, intermediate and evening types, respectively; there was no difference between them (Kruskal–Wallis H = 4.36; P = 0.113). [Table 2] shows the group differences in individual items of SRQ. Students who are evening oriented have more difficulties in making a decision as compared to other groups (P = 0.007), with small effect size (Cramer's V = 0.26). There was a trend toward evening and intermediate types of students becoming exhausted more easily (P = 0.089), with small effect size (Cramer's V = 0.18). In addition, evening oriented students showed the trend toward elevated vulnerability of feeling worthless than other chronotypes (P = 0.066), with small effect size (Cramer's V = 0.19).{Table 2}

Sleep parasomnias across chronotypes

[Table 3] shows sleep parasomnias among chronotypes. Among the students, 31 (20.7%) reported experiencing nightmares currently. The nightmare frequency ranged from 1 to 30 times/month, with a mean of 6.61 (SD 6.91) and a median of 4 times per month. The mean age of the first occurrence was 12.53 (SD 5.81) years. Among them, 9 (29%) reported factors precipitating these events; 4 (12.9%) reported stress as the reason, whereas one each (3.2%) reported family problems, hot weather, childhood sexual abuse or daily activities as related to the onset of nightmares. There was no difference in nightmare frequency between the chronotypes.{Table 3}

Night terror was reported by 21 (14%), the frequency ranged from 1 to 8 times/month, with a mean of 3.33 (SD 2.91) and a median of 2 times/month. The mean age of the first occurrence was 14.77 (SD 5.11) years and a median of 17 years. The mean duration of these episodes was 13.33 (SD 19.79) min and a median duration of 5 min. During these episodes, 9 (47.7%) had sweating, 13 (61.9%) had a rapid heartbeat, 3 (14.3%) had associated violent motor movements, and 13 (61.9%) reported recalling fragments of the dream preceding an episode. The most common dreams reported were being “chased by monster or ghost” (14.3%), “falling from height” (9.5%), and “death, betrayal, and disappointment” (9.5%). There was no difference in night terror frequency between the chronotypes.

Teeth grinding was reported to occur in 16 (10.7%) of the sample. The mean age of the first occurrence was 11.62 (SD 6.27) years and a median of 10.5 years. There was no difference in teeth grinding rates between the chronotypes. Forty-five (30%) reported wetting their bed during childhood. The reported mean age of occurrence was 2.25 (SD 1.59), and it occurred for a mean duration of 3.09 (SD 2.83) years. There was no difference in past bed wetting frequency between the chronotypes. Only 2 (1.3%) reported bedwetting that persisted during adulthood. Both of them had evening chronotype, and the association had a trend toward significance (P = 0.087) with a small effect size (Cramer's V = 0.215).

Eleven (7.4%) reported having sleepwalking episodes in the past, whereas only 3 (4.5%) reported recent sleepwalking. The mean age of the first occurrence was 10.81 (SD 2.82) years and a median of 11.5 years. There was no difference in sleepwalking between the chronotypes. 18 (12.2%) reported acting out their dreams during sleep. The mean age of the first occurrence was 10.41 (SD 6.21) years and a median of 12 years. There was no difference in the frequency of acting out their dreams between the chronotypes.

As substance use can affect parasomnias, the analysis of data was carried out after excluding subjects with substance use [Table 4]. However, the findings were similar to the complete sample. The current bedwetting was found in substance users, and none in the nonuser group had bedwetting.{Table 4}

Sleep quality across chronotypes

There was no difference in actual sleep per night and time to fall asleep between the chronotypes. However, Jonckheere-Terpstra test for the actual sleep time among chronotypes showed an increasing trend in sleeping hours from evening type to morning type (P = 0.055). Evening-oriented students showed a significantly higher frequency of initial insomnia (χ2 = 13.47, P = 0.036, Cramer's V = 0.21) and poorer overall sleep quality (χ2 = 12.92, P = 0.044, Cramer's V = 0.21) than the other groups. On the other hand, morning and evening types showed trouble staying awake than intermediate chronotypes (χ2 = 15.88, P = 0.014, Cramer's V = 0.23).


Distribution of chronotypes

The most common chronotype observed in our study was the intermediate type, seen in more than half of the samples (58%), followed by evening (22.7%) and morning types (19.3%). This was similar to a large sample study[26] on university students in the age range of 18–30 years, which found 15.84% morning and 24.54% evening types, and 59.62% intermediate types. In contrast, Paine et al.,[27] in their study in adults aged 30–49 years found 49.8% having morning type and only 5.6% having an evening type. However, further examination using cut offs suggested for middle-aged working adults by Taillard et al.,[28] 24.7% of the population was morning type and 26.4% was evening type. This suggests that evening chronotypes are more common in the younger population, and Horne and Ostberg classification[2] is useful in identifying the chronotypes among college students. The chronotypes were equally distributed across gender, unlike previous studies[29],[30] that found morningness more common in females and evening orientation more in males.

Our study found that evening chronotype was associated with a higher prevalence of alcohol and caffeine consumption and hypnotic use, which is similar to higher rates of substance use with evening chronotype in studies on college students.[5],[31],[32],[33],[34] In contrast to these studies, there was no association between evening orientation and smoking in our study. This may be related to lower rates of smoking among our sample, possibly due to increased awareness of its ill effects in medical students.

Psychological morbidity among chronotypes

In our study, the chronotypes did not differ on the psychological morbidity scores. However, “difficulty in decision making” was more common in evening types. Although higher consumption of caffeine-containing beverages was observed among adolescent evening types to reduce daytime sleepiness,[35] attention difficulties as a result of the irregular sleep-wake cycle, poorer sleep quality, and sleep deprivation were not relieved which may lead to difficulty in making a decision. In addition, evening type students likewise displayed increased vulnerability towards “feeling worthless and tired.” This is similar to other studies which showed elevated depressive symptoms and higher suicidal thoughts among evening-oriented students.[36],[37],[38],[39] It has been found that sleep deprivation in healthy evening chronotypes with normal sleep may actually reduce the depressive symptoms.[40] However, our study was done among medical students who had higher social and academic demands leading to less sleep than the general population.

Sleep problems among chronotypes

In our study, nightmares occurred in 20.7%, whereas night terror occurred in 14% subjects. Such high rates are reported in several large studies from other countries as well as from India.[41],[42],[43] The higher rates in these studies could reflect overestimation due to the use of questionnaires.

In our study, evening types had a higher frequency of initial insomnia and poorer sleep quality, similar to the previous studies.[44],[45] There was no significant association of sleep parasomnias with the chronotypes, except for the current bedwetting, which was seen in evening types. In contrast, previous studies have reported an association of nightmare with evening types, which may be a marker for associated psychopathology.[30],[46],[47] Nevertheless, persistent bed wetting in adulthood seen in evening chronotypes may resonate with the findings of the 18-year longitudinal study by Fichter et al.[14] which revealed increased sleep disturbances in childhood/adolescence were related to a higher risk of suffering from a psychiatric disorder in adulthood.

The limitation of our study was a cross-sectional design which can suggest only an association between sleeping problems, chronotypes, and mental health; longitudinal studies may shed light into the causality. Furthermore, the self-reported questionnaire may result in underreporting of psychopathology because of the associated stigma. There are possibilities of recall bias for childhood sleeping problems, and difficulty in differentiating night terrors from nightmares based on recollection has been reported to occur as they may be sometimes confused.[48] In addition, history of parasomnia is reliably obtained from bed or room partner; however, in our study, it was obtained as self-report, which could be an underestimation. This study was conducted on medical students, which limits generalization to other students. Furthermore, objective measures such as polysomnography could be used to study sleep parameters in those screening positive for sleep problems.


The most common chronotype was intermediate type in three-fifth and one-fifth each of evening and morning chronotypes. Persistent bedwetting was associated with evening chronotype, which could be an indicator of psychological morbidity. There was no significant difference for other parasomnias across chronotypes. Furthermore, the findings of this study suggest that poorer sleep quality and inadequate sleep among evening oriented students may lead to difficulty in making a decision and elevated susceptibility toward feeling worthlessness and tiredness. This indicates the importance of sleep education and the possibility of adapting daily routine to the special characteristics of different chronotypes to ensure maximum functioning of the students.

Financial support and sponsorship

Author NSW has received research grants from the Indian Council of Medical Research (ICMR) as part of Short Term Studentship (STS) in 2013.

Conflicts of interest

There are no conflicts of interest.


1Natale V, Cicogna P. Circadian regulation of subjective alertness in morning and evening 'types'. Perspect Individ Differ 1996;20:491-7.
2Horne JA, Ostberg O. A self-assessment questionnaire to determine morningness-eveningness in human circadian rhythms. Int J Chronobiol 1976;4:97-110.
3Duffy JF, Dijk DJ, Hall EF, Czeisler CA. Relationship of endogenous circadian melatonin and temperature rhythms to self-reported preference for morning or evening activity in young and older people. J Investig Med 1999;47:141-50.
4Baehr EK, Revelle W, Eastman CI. Individual differences in the phase and amplitude of the human circadian temperature rhythm: With an emphasis on morningness-eveningness. J Sleep Res 2000;9:117-27.
5Schneider ML, Vasconcellos DC, Dantas G, Levandovski R, Caumo W, Allebrandt KV, et al. Morningness-eveningness, use of stimulants, and minor psychiatric disorders among undergraduate students. Int J Psychol 2011;46:18-23.
6Roeser K, Meule A, Schwerdtle B, Kübler A, Schlarb AA. Subjective sleep quality exclusively mediates the relationship between morningness-eveningness preference and self-perceived stress response. Chronobiol Int 2012;29:955-60.
7Murray G, Harvey A. Circadian rhythms and sleep in bipolar disorder. Bipolar Disord 2010;12:459-72.
8Bullock B, Corlass-Brown J, Murray G. Eveningness and seasonality are associated with the bipolar disorder vulnerability trait. J Psychopathol Behav Assess 2014;36:446-51.
9Gelbmann G, Kuhn-Natriashvili S, Pazhedath TJ, Ardeljan M, Wöber C, Wöber-Bingöl C. Morningness: Protective factor for sleep-related and emotional problems in childhood and adolescence? Chronobiol Int 2012;29:898-910.
10Ivanenko A, Crabtree VM, Obrien LM, Gozal D. Sleep complaints and psychiatric symptoms in children evaluated at a pediatric mental health clinic. J Clin Sleep Med 2006;2:42-8.
11Fisher HL, Lereya ST, Thompson A, Lewis G, Zammit S, Wolke D. Childhood parasomnias and psychotic experiences at age 12 years in a United Kingdom birth cohort. Sleep 2014;37:475-82.
12Wong MM, Brower KJ, Fitzgerald HE, Zucker RA. Sleep problems in early childhood and early onset of alcohol and other drug use in adolescence. Alcohol Clin Exp Res 2004;28:578-87.
13Wong MM, Brower KJ, Nigg JT, Zucker RA. Childhood sleep problems, response inhibition, and alcohol and drug outcomes in adolescence and young adulthood. Alcohol Clin Exp Res 2010;34:1033-44.
14Fichter MM, Kohlboeck G, Quadflieg N, Wyschkon A, Esser G. From childhood to adult age: 18-year longitudinal results and prediction of the course of mental disorders in the community. Soc Psychiatry Psychiatr Epidemiol 2009;44:792-803.
15Gregory AM, Caspi A, Eley TC, Moffitt TE, Oconnor TG, Poulton R. Prospective longitudinal associations between persistent sleep problems in childhood and anxiety and depression disorders in adulthood. J Abnorm Child Psychol 2005;33:157-63.
16Gau SF, Soong WT. Psychiatric comorbidity of adolescents with sleep terrors or sleepwalking: A case-control study. Aust N Z J Psychiatry 1999;33:734-9.
17Hublin C, Kaprio J, Partinen M, Koskenvuo M. Nightmares: Familial aggregation and association with psychiatric disorders in a nationwide twin cohort. Am J Med Genet 1999;88:329-36.
18Levin R, Fireman G. Nightmare prevalence, nightmare distress, and self-reported psychological disturbance. Sleep 2002;25:205-12.
19Schredl M, Fricke-Oerkermann L, Mitschke A, Wiater A, Lehmkuhl G. Longitudinal study of nightmares in children: Stability and effect of emotional symptoms. Child Psychiatry Hum Dev 2009;40:439-49.
20Maughan B, Kim-Cohen J. Continuities between childhood and adult life. Br J Psychiatry 2005;187:301-3.
21Rutter M, Kim-Cohen J, Maughan B. Continuities and discontinuities in psychopathology between childhood and adult life. J Child Psychol Psychiatry 2006;47:276-95.
22Diaz-Morales JF, Prieto PD, Barreno CE, Mateo MC, Randler C. Sleep beliefs and chronotype among adolescents: The effect of a sleep education program. Biol Rhythm Res 2012;43:397-412.
23Beusenberd M, Orley J. A User's Guide to the Self Reporting Questionnaire; 1994. Available from: [Last accessed on 2013 Jun 23].
24Complete Sleep Solutions. Sleep Disorders Questionnaire – Adult; 2006. Available from: [Last accessed on 2013 Jun 23].
25Buysse DJ, Reynolds CF 3rd, Monk TH, Berman SR, Kupfer DJ. The Pittsburgh sleep quality index: A new instrument for psychiatric practice and research. Psychiatry Res 1989;28:193-213.
26Adan A, Natale V. Gender differences in morningness-eveningness preference. Chronobiol Int 2002;19:709-20.
27Paine SJ, Gander PH, Travier N. The epidemiology of morningness/eveningness: Influence of age, gender, ethnicity, and socioeconomic factors in adults (30-49 years). J Biol Rhythms 2006;21:68-76.
28Taillard J, Philip P, Chastang JF, Bioulac B. Validation of Horne and Ostberg morningness-eveningness questionnaire in a middle-aged population of French workers. J Biol Rhythms 2004;19:76-86.
29Putilov AA, Verevkin EG, Ivanova E, Donskaya OG, Putilov DA. Gender differences in morning and evening lateness. Biol Rhythm Res 2008;39:335-48.
30Nielsen T. Nightmares associated with the eveningness chronotype. J Biol Rhythms 2010;25:53-62.
31Hsu CY, Gau SS, Shang CY, Chiu YN, Lee MB. Associations between chronotypes, psychopathology, and personality among incoming college students. Chronobiol Int 2012;29:491-501.
32Robinson D, Gelaye B, Tadesse MG, Williams MA, Lemma S, Berhane Y. Daytime sleepiness, circadian preference, caffeine consumption and Khat use among college students in Ethiopia. J Sleep Disord: Treat Care 2013;3:1. doi: 10.4172/2325-9639.1000130.
33Whittier A, Sanchez S, Castañeda B, Sanchez E, Gelaye B, Yanez D, et al. Eveningness chronotype, daytime sleepiness, caffeine consumption, and use of other stimulants among Peruvian university students. J Caffeine Res 2014;4:21-7.
34Tran J, Lertmaharit S, Lohsoonthorn V, Pensuksan WC, Rattananupong T, Tadesse MG, et al. Daytime sleepiness, circadian preference, caffeine consumption and use of other stimulants among Thai college students. J Public Health Epidemiol 2014;8:202-10.
35Giannotti F, Cortesi F, Sebastiani T, Ottaviano S. Circadian preference, sleep and daytime behaviour in adolescence. J Sleep Res 2002;11:191-9.
36Gau SS, Shang CY, Merikangas KR, Chiu YN, Soong WT, Cheng AT. Association between morningness-eveningness and behavioral/emotional problems among adolescents. J Biol Rhythms 2007;22:268-74.
37Gaspar-Barba E, Calati R, Cruz-Fuentes CS, Ontiveros-Uribe MP, Natale V, De Ronchi D, et al. Depressive symptomatology is influenced by chronotypes. J Affect Disord 2009;119:100-6.
38Hidalgo MP, Caumo W, Posser M, Coccaro SB, Camozzato AL, Chaves ML. Relationship between depressive mood and chronotype in healthy subjects. Psychiatry Clin Neurosci 2009;63:283-90.
39Kitamura S, Hida A, Watanabe M, Enomoto M, Aritake-Okada S, Moriguchi Y, et al. Evening preference is related to the incidence of depressive states independent of sleep-wake conditions. Chronobiol Int 2010;27:1797-812.
40Selvi Y, Gulec M, Agargun MY, Besiroglu L. Mood changes after sleep deprivation in morningness-eveningness chronotypes in healthy individuals. J Sleep Res 2007;16:241-4.
41Suri JC, Sen MK, Adhikari T. Epidemiology of sleep disorders in the adult population of Delhi: A questionnaire based study. Indian J Sleep Med 2008;3:128-37.
42Santos-Silva R, Bittencourt LR, Pires ML, de Mello MT, Taddei JA, Benedito-Silva AA, et al. Increasing trends of sleep complaints in the city of Sao Paulo, Brazil. Sleep Med 2010;11:520-4.
43Munezawa T, Kaneita Y, Osaki Y, Kanda H, Ohtsu T, Suzuki H, et al. Nightmare and sleep paralysis among Japanese adolescents: A nationwide representative survey. Sleep Med 2011;12:56-64.
44Kabrita CS, Hajjar-Muça TA, Duffy JF. Predictors of poor sleep quality among Lebanese university students: Association between evening typology, lifestyle behaviors, and sleep habits. Nat Sci Sleep 2014;6:11-8.
45Yun JA, Ahn YS, Jeong KS, Joo EJ, Choi KS. The relationship between chronotype and sleep quality in Korean firefighters. Clin Psychopharmacol Neurosci 2015;13:201-8.
46Nadorff MR, Nazem S, Fiske A. Insomnia symptoms, nightmares, and suicidal ideation in a college student sample. Sleep 2011;34:93-8.
47Sheaves B, Porcheret K, Tsanas A, Espie CA, Foster RG, Freeman D, et al. Insomnia, nightmares, and chronotype as markers of risk for severe mental illness: Results from a student population. Sleep 2016;39:173-81.
48Wolke D. Feeding and sleeping across the lifespan. In: Rutter M, Hay D, editors. Development Through Life: A Handbook for Clinicians. Oxford: Blackwell Scientific Publications; 1994. p. 517-57.