Evening circadian preference is associated with sleep problems and daytime sleepiness in adolescents with ADHD

Adolescence is a developmental period characterized by disruptions in sleep and changes in circadian preferences. Although adolescents with attention‐deficit/hyperactivity disorder (ADHD) are at even higher risk of sleep disruption than their peers, no study has examined whether circadian preference is associated with sleep problems and daytime sleepiness in adolescents with ADHD. This study provides an initial test of the hypothesis that greater evening preference would be associated with more sleep problems and daytime sleepiness in adolescents diagnosed with ADHD. Participants were 80 adolescents (69% male), aged 13–17 years, with ADHD. Adolescents completed measures assessing circadian preference, pubertal development, anxiety/depressive symptoms and weeknight sleep duration. Both adolescents and parents completed measures of sleep problems and daytime sleepiness. In regression analyses controlling for a number of other variables (i.e., age, sex, pubertal development, ADHD medication use, and ADHD, oppositional defiant disorder and internalizing symptom severity), greater evening preference was associated with both adolescent‐ and parent‐reported sleep problems and daytime sleepiness. Greater evening preference remained significantly associated with each of these sleep problems and daytime sleepiness when also controlling for weeknight sleep duration. This is the first study to demonstrate that evening circadian preference is associated with both sleep problems and daytime sleepiness in adolescents with ADHD. The results indicate that it is important to consider circadian function as research examining sleep in adolescents with ADHD continues to advance.     

Polymorphisms in the circadian expressed genes PER3 and ARNTL2 are associated with diurnal preference and GNβ3 with sleep measures

Sleep and circadian rhythms are intrinsically linked, with several sleep traits, including sleep timing and duration, influenced by both sleep homeostasis and the circadian phase. Genetic variation in several circadian genes has been associated with diurnal preference (preference in timing of sleep), although there has been limited research on whether they are associated with other sleep measurements. We investigated whether these genetic variations were associated with diurnal preference (Morningness–Eveningness Questionnaire) and various sleep measures, including: the global Pittsburgh Sleep Quality index score; sleep duration; and sleep latency and sleep quality. We genotyped 10 polymorphisms in genes with circadian expression in participants from the G1219 sample (n = 966), a British longitudinal population sample of young adults. We conducted linear regressions using dominant, additive and recessive models of inheritance to test for associations between these polymorphisms and the sleep measures. We found a significant association between diurnal preference and a polymorphism in period homologue 3 (PER3) (P < 0.005, recessive model) and a novel nominally significant association between diurnal preference and a polymorphism in aryl hydrocarbon receptor nuclear translocator‐like 2 (ARNTL2) (P < 0.05, additive model). We found that a polymorphism in guanine nucleotide binding protein beta 3 (GNβ3) was associated significantly with global sleep quality (P < 0.005, recessive model), and that a rare polymorphism in period homologue 2 (PER2) was associated significantly with both sleep duration and quality (P < 0.0005, recessive model). These findings suggest that genes with circadian expression may play a role in regulating both the circadian clock and sleep homeostasis, and highlight the importance of further studies aimed at dissecting the specific roles that circadian genes play in these two interrelated but unique behaviours.     

Improvement of a patient's circadian rhythm sleep disorders by aripiprazole was associated with stabilization of his bipolar illness

Splitting of the behavioural activity phase has been found in nocturnal rodents with suprachiasmatic nucleus (SCN) coupling disorder. A similar phenomenon was observed in the sleep phase in the diurnal human discussed here, suggesting that there are so‐called evening and morning oscillators in the SCN of humans. The present case suffered from bipolar disorder refractory to various treatments, and various circadian rhythm sleep disorders, such as delayed sleep phase, polyphasic sleep, separation of the sleep bout resembling splitting and circabidian rhythm (48 h), were found during prolonged depressive episodes with hypersomnia. Separation of sleep into evening and morning components and delayed sleep‐offset (24.69‐h cycle) developed when lowering and stopping the dose of aripiprazole (APZ). However, resumption of APZ improved these symptoms in 2 weeks, accompanied by improvement in the patient's depressive state. Administration of APZ may improve various circadian rhythm sleep disorders, as well as improve and prevent manic–depressive episodes, via augmentation of coupling in the SCN network.     

GP78基因多态性与冠心病相关

目的探讨GP78基因多态性与冠心病的相关性。方法 Taq Man SNP基因分型法对1109例(557例冠心病患者+552例对照者)无血缘关系的研究对象进行基因分型,并应用病例对照的研究方法进行相关性分析。结果GP78基因rs2617849位点的基因分型和等位基因频率在冠心病组和对照组之间的分布存在明显差异(P0.05),冠心病组携带TT基因型(TT vs CC+CT)和T等位基因高于对照组(P0.05),在排除吸烟、高血压、糖尿病和低密度脂蛋白胆固醇等混杂因素后,仍存在显著性差异(95%CI:1.035~1.736,P0.05)。结论 GP78基因的rs2617849位点多态性与冠心病相关,rs2617849的TT基因型和T等位基因可作为冠心病易感基因标记。     

Effects of exogenous melatonin on sleep and circadian rhythms in women with premenstrual dysphoric disorder

We previously found normal polysomnographic (PSG) sleep efficiency, increased slow-wave sleep (SWS), and a blunted melatonin secretion in women with premenstrual dysphoric disorder (PMDD) compared to controls. Here, we investigated the effects of exogenous melatonin in five patients previously studied. They took 2 mg of slow-release melatonin 1 h before bedtime during their luteal phase (LP) for three menstrual cycles. At baseline, patients spent every third night throughout one menstrual cycle sleeping in the laboratory. Measures included morning urinary 6-sulfatoxymelatonin (aMt6), PSG sleep, nocturnal core body temperature (CBT), visual analog scale for mood (VAS-Mood), Prospective Record of the Impact and Severity of Menstrual Symptoms (PRISM), and ovarian plasma hormones. Participants also underwent two 24-hour intensive physiological monitoring (during the follicular phase and LP) in time-isolation/constant conditions to determine 24-hour plasma melatonin and CBT rhythms. The same measures were repeated during their third menstrual cycle of melatonin administration. In the intervention condition compared to baseline, we found increased urinary aMt6 (p < 0.001), reduced objective sleep onset latency (p = 0.01), reduced SWS (p < 0.001), and increased Stage 2 sleep (p < 0.001). Increased urinary aMt6 was correlated with reduced SWS (r = −0.51, p < 0.001). Circadian parameters derived from 24-hour plasma melatonin and CBT did not differ between conditions, except for an increased melatonin mesor in the intervention condition (p = 0.01). Ovarian hormones were comparable between the conditions (p ≥ 0.28). Symptoms improved in the intervention condition, as measured by the VAS-Mood (p = 0.02) and the PRISM (p < 0.001). These findings support a role for disturbed melatonergic system in PMDD that can be partially corrected by exogenous melatonin.     

Length polymorphism in the Period 3 gene is associated with sleepiness and maladaptive circadian phase in night‐shift workers

The objective of the current study was to determine if night‐shift workers carrying the five‐repeat variant of the Period 3 gene show elevated levels of nocturnal sleepiness and earlier circadian phase compared with homozygotes for the four‐repeat allele. Twenty‐four permanent night‐shift workers were randomly selected from a larger study. Participants took part in an observational laboratory protocol including an overnight multiple sleep latency test and half‐hourly saliva collection for calculation of dim‐light melatonin onset. Period 3^–/5 shift workers had significantly lower multiple sleep latency test during overnight work hours compared with Period 3^4/4 workers (3.52 ± 23.44 min versus 10.39 ± 6.41 min, P = 0.003). We observed no significant difference in sleepiness during early morning hours following acute sleep deprivation. Long‐allele carriers indicated significantly higher sleepiness on the Epworth Sleepiness Scale administered at 17:00 hours (12.08 ± 2.55 versus 8.00 ± 1.94, P < 0.001). We observed a significantly earlier melatonin onset in Period 3^–/5 individuals compared with Period 3^4/4 shift workers (20:44 ± 6:37 versus 02:46 ± 4:58, P = 0.021). Regression analysis suggests that Period 3 genotype independently predicts sleepiness even after controlling for variations in circadian phase, but we were unable to link Period 3 to circadian phase when controlling for sleepiness. Period 3^–/5 shift workers showed both subjective and objective sleepiness in the pathological range, while their Period 3^4/4 counterparts showed sleepiness within normal limits. Period 3^–/5 night workers also show a mean circadian phase 6 h earlier (i.e. less adapted) than Period 3^4/4 workers. Because Period 3^–/5 workers have maladaptive circadian phase as well as pathological levels of sleepiness, they may be at greater risk for occupational and automotive accidents. We interpret these findings as a call for future research on the role of Period 3 in sleepiness and circadian phase, especially as they relate to night work.     

The 24-h growth hormone rhythm in men: sleep and circadian influences questioned

The 24-h rhythm of growth hormone (GH) is thought to be controlled primarily by sleep processes with a weak circadian component. This concept has been recently questioned in sleep-deprived persons. To test the notion of a high sleep-dependency of GH release, we established simultaneous 24-h rhythms of GH and melatonin, a circadian marker, in night workers who form a model for challenging sleep and circadian processes. Ten day-active subjects and 11 night workers were studied during their usual sleep-wake schedule, with sleep from 23:00 to 07:00 hours and 07:00 to 15:00 hours, respectively. Experiments were conducted in sleep rooms under continuous nutrition, bed rest, and dim light. Melatonin and GH were measured every 10 min over 24 h. In day-active subjects, melatonin and GH showed the well-known 24-h profiles, with a major sleep-related GH pulse accounting for 52.8 +/- 3.5% of the 24-h GH production and the onset of the melatonin surge occurring at 21:53 hours +/- 18 min. In night workers, melatonin showed variable circadian adaptation, with the onset of secretion varying between 21:45 and 05:05 hours. The sleep-related GH pulse was lowered, but the reduction was compensated for by the emergence of large individual pulses occurring unpredictably during waking periods, so that the total amount of GH secreted during the 24 h was constant. One cannot predict the degree of GH adaptation from the highly variable melatonin shift. These results argue against the concept that sleep processes exert a predominant influence on GH release whatever the conditions. When sleep and circadian processes are misaligned, the blunting of the sleep-related GH pulse is counteracted, as in sleep-deprived persons, by a compensatory mechanism promoting GH pulses during wakefulness.     

A promoter polymorphism in the CD14 gene is associated with elevated levels of soluble CD14 but not with IgE or atopic diseases

BACKGROUND: A polymorphism in the promoter region of the CD14 gene, C-159T, has been shown to be associated with increased levels of soluble CD14 (sCD14) and decreased serum immunoglobulin E (IgE) and the expression of a more severe atopic phenotype in previous studies. METHODS: To test if these associations are consistently found in different populations and different age groups, we genotyped 2048 children of different age groups as well as 888 adults from different regions of Germany for the CD14 C-159T polymorphism. RESULTS: While an association between this promoter polymorphism and levels of sCD14 could be confirmed in our study population (CC: 1017 ng/ml vs TT: 1370 ng/ml, P = 0.03), no association between CD14 C-159T genotypes and IgE levels or the prevalence of atopic diseases was seen. CONCLUSIONS: The lack of association between CD14 genotypes and IgE as well as atopic outcomes in this large German study population seems to indicate that CD14 genotypes may not directly be involved in the development of allergies during childhood.     

Chronotype is associated with the timing of the circadian clock and sleep in toddlers

Chronotype is a construct reflecting individual differences in diurnal preference. Although chronotype has been studied extensively in school‐age children, adolescents and adults, data on young children are scarce. This study describes chronotype and its relationship to the timing of the circadian clock and sleep in 48 healthy children aged 30–36 months (33.4 ± 2.1 months; 24 males). Parents completed the Children's Chronotype Questionnaire (CCTQ) ~2 weeks before the start of the study. The CCTQ provides three measures of chronotype: midsleep time on free days, a multi‐item morningness/eveningness score and a single item chronotype score. After 5 days of sleeping on their habitual schedule (assessed with actigraphy and sleep diaries), children participated in an in‐home salivary dim light melatonin onset assessment. Average midsleep time on free days was 1:47 ± 0:35, and the average morningness/eveningness score was 26.8 ± 4.3. Most toddlers (58.4%) were rated as ‘definitely a morning type’ or ‘rather morning than evening type’, while none (0%) were rated as ‘definitely evening type’. More morning types (midsleep time on free days and morningness/eveningness score, respectively) had earlier melatonin onset times (r = 0.45, r = 0.26), earlier habitual bedtimes (r = 0.78, r = 0.54), sleep onset times (r = 0.80, r = 0.52), sleep midpoint times (r = 0.90, r = 0.53) and wake times (r = 0.74, r = 0.34). Parent ratings using the single‐item chronotype score were associated with melatonin onset (r = 0.32) and habitual bedtimes (r = 0.27), sleep onset times (r = 0.33) and sleep midpoint times (r = 0.27). Morningness may best characterize circadian preference in early childhood. Associations between chronotype and circadian physiology and sleep timing suggest adequate validity for the CCTQ in this age group. These findings have important implications for understanding the marked variability in sleep timing during the early years of life.     

Association of job stress,CLOCK gene polymorphism and their interaction with poor sleep quality

Job stress and the Circadian Locomotor Output Cycles Kaput (CLOCK) gene could affect circadian rhythm and sleep quality. The main aim of our present study was to investigate the association of job stress, CLOCK gene polymorphism and their interaction with sleep quality in a non‐clinical Chinese Han population, which has not been reported to date. Using a cross‐sectional design, 450 subjects were recruited in Beijing. Sleep quality was measured with the Pittsburgh Sleep Quality Index (PSQI) and job stress was measured with the Work Stress Scale. CLOCK gene rs11932595 polymorphism was genotyped in 297 blood samples. Correlation analysis showed a close but different association of high job stress with the PSQI and its components. Analysis of variance showed significant main effects of the CLOCK gene rs11932595 polymorphism. G‐allele carriers had a higher score in the PSQI, sleep duration, sleep latency and sleep disturbances. Further interaction analyses showed an ordinal interaction on sleep duration, and a disordinal interaction on daytime dysfunction. Specifically, G‐allele carriers had poorer sleep duration than AA homozygotes when in high job stress, while the two subgroups displayed similar sleep duration when in low job stress, conforming to the diathesis–stress model. In comparison to G‐allele carriers, AA homozygotes experienced less daytime dysfunction when in low job stress whereas more daytime dysfunction when in high job stress, fitting with the differential susceptibility model. As genetic links have been revealed, our investigation might be conducive for elucidating aetiological factors for sleep quality and targets for implementing interventions to attain good sleep quality.     

Cysteinyl leukotriene receptor 1 promoter polymorphism is associated with aspirin-intolerant asthma in males

BACKGROUND: Cysteinyl leukotrienes (CysLTs) play important roles in the pathogenesis of eosinophilic airway inflammation characterized by bronchoconstriction, mucus secretion and airway hyper-responsiveness via cysteinyl leukotriene receptor 1 (CysLTR1)-mediated mechanism. CysLTR1-selective antagonists have anti-bronchoconstrictive and anti-inflammatory effects in asthma, particularly aspirin-intolerant asthma (AIA). METHODS: To investigate the association of CysLTR1 with AIA development, we identified three single nucleotide polymorphisms (SNPs), -634C>T, -475A>C, -336A>G, in the 5' upstream region of CysLTR1 gene using a direct sequencing method in 105 AIA patients, 110 ASA-tolerant asthma (ATA) patients and 125 normal healthy controls (NC). RESULTS: Significant differences were observed in allele frequencies of the three SNPs within male subjects; Male AIA patients had higher frequencies of the minor alleles of these three SNPs than male control groups (P=0.03 for AIA vs. NC; P=0.02 for AIA vs. ATA). Moreover, three-SNP haplotype, ht2 [T-C-G], was associated with increased disease risk (odds ratio (OR)=2.71, P=0.03 for AIA vs. NC; OR=2.89, P=0.02 for AIA vs. ATA) in males. CysLTR1 haplotypes were also associated with altered gene expression; luciferase activity was significantly enhanced with the ht2 [T-C-G] construct in comparison with the ht1 [C-A-A] construct in human Jurkat cells (P=0.04). CONCLUSION: These results suggest that genetic variants of CysLTR1 are associated with AIA in a Korean population, and may modulate CysLTR1 expression.     

Diurnal preference, sleep habits, circadian sleep propensity and melatonin rhythm in healthy human subjects

After 24-h sleep deprivation, 33 healthy young subjects entered the 10/20 min ultra-short sleep–wake schedule for 26 h. Melatonin rhythm was hourly assessed simultaneously. Results indicated that morning preference was significantly correlated with habitual sleep onset (r=−0.41, P=0.04), habitual sleep offset (r=−0.52, P=0.002), melatonin peak time (r=−0.36, P=0.04), and sleep propensity onset time (r=−0.36, P=0.04). The intervals between habitual sleep mid-point and melatonin peak time and between habitual sleep mid-point and sleep propensity onset time were significantly longer in morning-preference subjects than in evening-preference subjects (P<0.05). These findings suggest that the variance of diurnal preference may be related to differences in phase relations between habitual sleep timing and the circadian pacemaker.     

Genetic variation of the PSCA gene (rs2294008) is not associated with the risk of prostate cancer

Prostate stem cell antigen (PSCA) is a cell-membrane glycoprotein consisting of 123 amino acids and highly expressed in the prostate, but there have been few reports on the relationship between rs2294008 of PSCA and prostate cancer in the literature. Therefore, we evaluated the association between rs2294008 and the risk of prostate cancer. A total of 240 prostate cancer patients and 306 controls (patients with benign prostatic hyperplasia) were enrolled. Genotype analysis of rs2294008 of PSCA was performed using PCR. Logistic regression analysis was performed according to the genotype of PSCA rs2294008. We found that CT and TT genotypes were associated with an insignificant risk of prostate cancer compared with the CC genotype (P = 0.627 and 0.397, respectively). In addition, there was no significant difference in rs2294008 according to clinicopathological parameters, such as age, Gleason score, prostate-specific antigen (PSA), stage, and metastasis in prostate cancer (P > 0.05 for each). Age, Gleason score, PSA, pathologic stage, and metastasis did not modify the association between PSCA and the risk of prostate cancer (each P > 0.05 for each). Taken together, the genetic polymorphism of PSCA rs2294008 was not associated with the risk of prostate cancer. Our results suggest that rs2294008 may not play a role in prostate carcinogenesis.     

Alertness, mood and performance rhythm disturbances associated with circadian sleep disorders in the blind

Blind people report disturbances in alertness, mood and performance. In laboratory studies, these waking functions can only be maintained when the wake-dependent deterioration is opposed by appropriately-timed endogenous circadian rhythms. We aimed to quantify whether variations in waking function experienced by blind people living in society were dependent on the phase relationship between the sleep-wake cycle and the circadian pacemaker. The time course of alertness, mood and performance was assessed in 52 blind subjects with and without circadian rhythm disorders every 2 h for 2 days per week for 4 weeks. Sleep-wake timing and circadian phase were assessed from diaries and weekly measurements of urinary 6-sulphatoxymelatonin rhythms, respectively. In those subjects who woke at either a normal circadian phase (n = 26) or abnormally early (n = 5), alertness, mood and performance deteriorated significantly with increased time awake (P < 0.05). In 17 non-entrained ('free-running') subjects, waking function varied significantly with circadian phase such that subjects rated themselves most sleepy (P = 0.03) and most miserable (P = 0.02) when they were awake during the time of peak melatonin production. The internal phase relationship between sleep-wake behaviour and the circadian melatonin rhythm in entrained subjects contributed to predictable differences in the daily profile of alertness, mood and performance. Disruption of this phase relationship in non-entrained blind individuals with circadian rhythm sleep disorders resulted in impaired waking function during the day equivalent to that usually only experienced when awake during the night. Treatment for circadian rhythm disorders should be targeted in normalizing these phase relationships.     

A silent polymorphism in the PER1 gene associates with extreme diurnal preference in humans

The three PERIOD proteins form a major negative feedback component of the molecular mechanism governing the periodicity of the vertebrate circadian clock. Genetic variations within the human PER2 and PER3 genes have been linked with diurnal preference and disorders of sleep timing. We screened the coding region of PER1, as well as the 5′- and 3′-untranslated regions and the promoter region, for polymorphisms. The T2434C polymorphism in exon 18, a synonymous substitution, associated with extreme diurnal preference. The C allele was more frequent in subjects with extreme morning preference (frequency = 0.24) than in subjects with extreme evening preference (frequency = 0.12). No significant association was observed between either allele and delayed sleep phase syndrome. This polymorphism may have a direct effect on RNA translatability, or be in linkage disequilibrium with another polymorphism which affects PER1 expression at the DNA, RNA, or protein level. This is the first reported association between a PER1 polymorphism and extreme diurnal preference. Functionally important polymorphisms in PER1 are rare, which may indicate that it is subject to more stringent selection pressure than the other PER genes.     

Objective measures of sleep and dim light melatonin onset in adolescents and young adults with delayed sleep phase disorder compared to healthy controls

Delayed sleep phase disorder is characterized by a delay in the timing of the major sleep period relative to conventional norms. The sleep period itself has traditionally been described as normal. Nevertheless, it is possible that sleep regulatory mechanism disturbances associated with the disorder may affect sleep duration and/or architecture. Polysomnographic data that may shed light on the issue are scarce. Hence, the aim of this study was to examine polysomnographic measures of sleep in adolescents and young adults with delayed sleep phase disorder, and to compare findings to that of healthy controls. A second aim was to estimate dim light melatonin onset as a marker of circadian rhythm and to investigate the phase angle relationship (time interval) between dim light melatonin onset and the sleep period. Data from 54 adolescents and young adults were analysed, 35 diagnosed with delayed sleep phase disorder and 19 healthy controls. Results show delayed timing of sleep in participants with delayed sleep phase disorder, but once sleep was initiated no group differences in sleep parameters were observed. Dim light melatonin onset was delayed in participants with delayed sleep phase disorder, but no difference in phase angle was observed between the groups. In conclusion, both sleep and dim light melatonin onset were delayed in participants with delayed sleep phase disorder. The sleep period appeared to occur at the same circadian phase in both groups, and once sleep was initiated no differences in sleep parameters were observed.     

TLR-2 gene Arg753Gln polymorphism is strongly associated with acute rheumatic fever in children

The recently described family of toll-like receptors (TLRs) is a key player in host immunity by mediating inflammatory reactions against a wide range of pathogens. Mutations and polymorphisms in TLRs have revealed the importance of TLRs in human defence against diseases. TLR-2 is reported to interact with different bacterial structures, including lipoproteins, peptidoglycan and lipoteichoic acid. To assess the role of TLR-2 gene polymorphism in acute rheumatic fever (ARF) etiopathology, 61 independent Caucasian Turkish patients and 91 child and 116 adult controls were studied. Antistreptolycin O, C-reactive protein, sedimentation and white blood cell counts were studied to evaluate the clinical characteristics of the patients. Genomic DNA was extracted from peripheral blood using a standard column extraction technique. The Arg753Gln and Arg677Trp polymorphisms were genotyped by polymerase chain reaction (PCR) restriction fragment length polymorphism. The PCR products for the TLR-2 gene were analysed on 1.5% agarose gel pre-stained with ethidium bromide. Compared with healthy adult controls, the Arg753Arg genotype was significantly decreased in the entire group of ARF cases [odds ratio (OR) 0.01, 95% confidence interval (95% CI) 0.0034–0.031, p<0.0001]. Significantly, ARF patients were just 16 times more frequent with Gln allele (OR 15.6, 95% CI 7.87–30.8, p<0.0001). Moreover, evidence for an intensifying effect of the Gln allele was noteworthy when patients with Arg753Gln genotype were compared with healthy controls (OR 97.1, 95% CI 32.5–290, p<0.0001). However, no Arg677Trp polymorphism was detected in either patients or controls. Our data suggest that there is strong evidence for the biological role of TLR-2 in ARF. The common TLR-2 Arg to Gln polymorphism at position 753 significantly contributes to the pathogenesis of ARF. These results will allow the construction of a profile of individuals prone to ARF and may assist in developing new therapies.     

Age-related change in the association between a polymorphism in the PER3 gene and preferred timing of sleep and waking activities

The objective of this study was to investigate the effect of age on the association between preferred timing of sleep and waking activities and a coding-region variable number tandem repeat (VNTR) polymorphism in the clock gene PER3. We have previously reported this polymorphism to associate with diurnal preference and delayed sleep phase syndrome (DSPS). Participants (n = 1590; 707 males and 883 females) completed the Horne-Ostberg (HO) questionnaire for diurnal preference and provided a DNA sample. Overall HO scores were plotted against age. The 5% extremes and intermediates were selected for genotyping. Frequencies of the PER3 4- and 5-repeat alleles were examined in separate age groups (18-29, 30-39, 40-49 and 50+ years of age). The 4-repeat allele was significantly more frequent in evening types, and the 5-repeat allele more frequent in morning types (Fisher's exact test, P = 0.016). Analysis in the four age groupings revealed that the strength of this association attenuated with age and was significant only in the youngest group (18-29 years). These results extend our previous finding of an association between the PER3 VNTR and diurnal preference. They also demonstrate that diurnal preference in young people is more closely associated with this polymorphism than it is in other age groups.