افراد سحرخیز، افراد پایداری هستند: ریتم شبانه روزی و عوامل مرتبه بالاتر از پنج عامل بزرگ

A personality model based on the Big Five and their higher-order factors or metatraits was used to examine associations between personality and individual differences in circadian rhythm, as assessed by the Morningness–Eveningness Questionnaire (MEQ). Based on previous research with Eysenck’s personality model and a neurobiological model implicating serotonergic function in the metatrait Stability (the shared variance of Neuroticism reversed, Agreeableness, and Conscientiousness), we hypothesized that morningness would be positively related to Stability. Structural equation modeling in a sample of 279 undergraduates confirmed this hypothesis.

Some people are early birds; others are night owls – morning people and evening people. This is a common lay observation, but there is also scientific evidence for the validity of these classifications and, additionally, for the existence of people who prefer the middle of the day to either morning or evening. Preferences for time of waking and sleeping, as well as for time of day for accomplishing demanding intellectual and physical tasks, can be reliably measured and appear to have a biological basis. Like most organisms, human beings show circadian rhythms in many behavioral and biological variables. When not exposed to environmental cues providing temporal information, the human circadian cycle has a free-running period of about 25 h. Normally, however, it is entrained to a 24 h period, primarily through exposure to the daily cycle of light and dark (Miller, Morin, Schwartz, & Moore, 1996). Like most characteristics of organisms, circadian rhythm is subject to individual variation. Barring extenuating circumstances, people feel most alert, energetic, and capable at a particular time of day, which varies from person to person but remains reasonably stable in a given individual (although there are regular changes over the lifespan – during early adolescence, for example, peak arousal typically shifts from morning toward later in the day; Kim, Dueker, Hasher, & Goldstein, 2002). These stable differences in time of peak arousal appear to be responsible for the existence of morning people, evening people, and middle-of-the-day people. Given the importance of circadian rhythms in human functioning (they regulate sleep, appetite, and cognitive function, among other things), it is of interest to know whether their variation is associated with personality more generally. A number of studies have examined associations between time of peak arousal and Eysenck’s three personality dimensions, Extraversion, Neuroticism, and Psychoticism. Most attention has been paid to Extraversion because Eysenck originally hypothesized that cortical arousal was the biological factor linked to variation in Extraversion (Eysenck, 1967). Results have been mixed. In a review of 30 years of research on individual differences in circadian rhythms, Tankova, Adan, and Buela-Casal (1994) reported 15 studies examining Extraversion, 11 of which also examined Neuroticism, and two of which also examined Psychoticism. Nine of these studies found a significant association between eveningness (late peak arousal) and Extraversion, and two more reported trends in that direction. Four studies reported a significant association between eveningness and Neuroticism, and one more reported a trend in that direction. Hess, Sherman, and Goodman (2000) demonstrated an association between eveningness and Neuroticism and cited one additional study not covered in Tankova and colleagues’ review that also found this association (Mura & Levy, 1986). Finally, the two reviewed studies that examined Psychoticism found it to be significantly associated with eveningness. One later study found associations between eveningness and both Psychoticism and Extraversion, but not Neuroticism (Mitchell & Redman, 1993), and another found associations between eveningness and both Psychoticism and Neuroticism, but not Extraversion (Mecacci & Rocchetti, 1998). Lateness of peak arousal, therefore, may be associated with Extraversion, Neuroticism, and Psychoticism, but the number of null results suggests caution in drawing conclusions. The apparent association between Extraversion and circadian rhythm is complicated by the history of Eysenck’s personality model, which originally contained only two factors, Extraversion and Neuroticism (Eysenck, 1947). When Psychoticism was added to the model (Eysenck & Eysenck, 1975), the trait of impulsivity was moved from Extraversion to Psychoticism (though Extraversion retained “venturesomeness” and “sensation seeking”, which Eysenck deemed related to impulsivity), and the Eysenck Personality Inventory (EPI) was redesigned accordingly, becoming the Eysenck Personality Questionnaire (EPQ). Most of the studies cited by Tankova and colleagues utilized the EPI, thereby confounding Extraversion and Psychoticism. Eysenck himself suggested that the impulsivity dimension of Extraversion was likely to be responsible for individual differences in arousal (Eysenck & Folkard, 1980). Based on this suggestion and the results of the few studies that divided Extraversion into subdimensions of impulsivity and sociability, Tankova et al. (1994) concluded that impulsivity was likely to be responsible for the positive associations found between eveningness and Extraversion. Given this situation, circadian rhythm may be more likely to be related to Psychoticism than to Extraversion. The present study attempted to integrate and clarify past findings and to provide a more comprehensive assessment of the associations between circadian rhythm and personality, by using a hierarchical model of personality based on the Big Five and their higher-order factors (DeYoung, 2006 and DeYoung et al., 2002). Over the past 20 years, the Five Factor Model or Big Five, which divides personality traits into five broad domains (Neuroticism, Agreeableness, Conscientiousness, Extraversion, and Openness/Intellect), has become one of the most widely used taxonomies of personality (Costa and McCrae, 1992, Digman, 1990 and John and Srivastava, 1999). Fortunately for the sake of integration, the Big Five are not incompatible with Eysenck’s dimensions. Extraversion and Neuroticism remain very similar in both models, and Eysenck’s misleadingly named Psychoticism corresponds to a combination of low Agreeableness and low Conscientiousness (Eysenck, 1992 and Goldberg and Rosolack, 1994). Openness/Intellect primarily reflects individual differences in cognitive functioning (DeYoung et al., 2005 and Pytlik Zillig et al., 2002), which Eysenck excluded from his model because he felt them to be the domain of intelligence testing (though Openness/Intellect is a broader construct than intelligence; DeYoung et al., 2005 and McCrae and Costa, 1997). Although the Big Five were originally conceived as orthogonal factors and the highest level of a taxonomy of trait descriptors, they have proven to be regularly intercorrelated and to demonstrate a consistent higher-order factor solution (DeYoung, 2006, DeYoung et al., 2002 and Digman, 1997). Neuroticism (reversed), Agreeableness, and Conscientiousness form a first factor, labeled Stability, while Extraversion and Openness/Intellect form a second, labeled Plasticity ( DeYoung et al., 2002). 1 Stability is evident in emotional (Neuroticism), social (Agreeableness), and motivational (Conscientiousness) domains. Plasticity denotes an exploratory tendency in both behavioral (Extraversion) and cognitive (Openness/Intellect) modalities. The higher-order factors have been dubbed “metatraits” ( Digman, 1997) and may provide a useful starting point for the development of a psychobiological model of personality based on the Big Five ( DeYoung, 2006 and DeYoung et al., 2002). Evidence suggests that Stability is associated with variability in serotonergic function while Plasticity is associated with variability in dopaminergic function ( DeYoung, 2006, DeYoung et al., 2002 and DeYoung et al., 2005). This neurobiological model is of potential relevance to the link between circadian rhythm and personality because serotonin is heavily implicated in the control of circadian rhythm. The brain’s primary clock mechanism is the suprachiasmatic nucleus (SCN) in the anterior hypothalamus, and its three major afferent pathways are from the retina, the intergeniculate leaflet, and the midbrain serotonergic system (Miller et al., 1996). Serotonergic inputs to the SCN modulate the entrainment of circadian rhythms to light and also appear to mediate activity-induced shifts in circadian rhythm (Miller et al., 1996, Mistleberger et al., 2000 and Yuan et al., 2005). Serotonin may be responsible for stabilizing circadian rhythms, making them less likely to shift in response to light exposure during what would normally be the dark half of the daily cycle (e.g., from electric lights in the evening) (Yuan et al., 2005). Given the putative link between serotonergic function and the personality trait Stability, one might expect Stability to be related to individual differences in circadian rhythm, with individuals higher in Stability showing higher levels of morningness. After translating from Eysenck’s model to the Big Five, the personality associations reviewed above are consistent with our hypothesis regarding Stability. Because the few examinations of Psychoticism in relation to circadian rhythm have all found it to be associated with eveningness, morningness (early peak arousal) should be associated with Agreeableness and Conscientiousness. Additional evidence to suggest an association between Conscientiousness and morningness comes from a study of sleep habits (Gray & Watson, 2002), which did not include a direct measure of circadian rhythm, but did find that Conscientiousness was associated with sleep schedule, such that conscientious individuals both went to bed and awoke earlier. Taken with the sporadic findings of association between Neuroticism and eveningness, the likely link with Agreeableness and Conscientiousness suggests that morningness might most accurately be considered a correlate of the metatrait Stability. The inconsistent findings with Neuroticism would be more explicable if it were the case that only the variance that Neuroticism shares with Agreeableness and Conscientiousness was associated with morningness. Additional evidence that the association with Neuroticism may be valid comes from studies showing that depression is associated with eveningness (Chelminski et al., 1999 and Drennan et al., 1991). In the Five Factor Model, depression is a facet of Neuroticism (Costa & McCrae, 1992). In sum, the variety of associations found between personality and circadian rhythm suggests that the metatrait level of personality structure may be the most appropriate and parsimonious level at which to examine their interrelation. In the present study, we used structural equation modeling to examine the associations between morningness and the metatraits. We hypothesized that Stability would be positively related to morningness. Little evidence exists to suggest any association between circadian rhythm and Plasticity, particularly given doubt about the association of eveningness with Extraversion (resulting from Eysenck’s initial conflation of Psychoticism and Extraversion).