Being diurnal creatures, humans usually sleep at night and are active throughout the day. However, industrialization has quickly altered sleep-wake patterns, resulting in excessive nighttime exposure to artificial light, television, and smartphones, irregular lifestyles, innovative eating habits, and a rise in the usage of stimulants like coffee in contemporary society. As a result, people now lead unpredictable lives, work shifts, adopt unusual eating patterns, and drink more coffee and other stimulants [1-4]. Due to social or occupational factors, people have been under tremendous pressure to modify their nocturnal rhythms in order to adapt to current lifestyles and trends. For all, such as healthcare professionals, drivers, and essential service providers, this causes a discrepancy between their professional obligations and their need for sleep, which is contributing to the rise of the chronically sleep-deprived world.

Taillard et al. [5] proposed that an individual’s timing of internal circadian rhythms, directed by the circadian clock, may be in sync or out of sync with their sleep timings based on their daily social interactions. They further suggestedthat a person’s preferred sleep schedule and time may be influenced by social factors. We refer to these sleep schedules or preferences that deviate from biological time as circadian disturbances. Furthermore, everyone has a different preference for sleep or wakefulness in a sleep-deprived world [5]. Over the past few decades, interest has steadily grown in the significance of diurnal preference and chronotype, and how disruptions caused by social factors affect our internal clock [6] but also has significant comorbidity with psychiatric diseases [7], neurodevelopmental disorders [8], cognitive dysfunction, and aberrant emotional processing [9].

In this context, it is crucial to ask if and to what degree a person’s chronotype and individual necessity or preference for sleep interact to affect different aspects of their physical and mental well-being, encompassing the framework and function of the brain. Prior to delving into the solution, it is advisable to determine the most thorough and practical model and chronotype metrics that may be applied in a worldwide study setting.

A diverse set of biological rhythms, which are cyclic changes in bodily chemicals or functions, can be observed in humans. These rhythms can occur multiple times a day, once every 24 hours, or take weeks to complete. These rhythms are found at various levels, from single cells to social behavior, and nearly all physiological and psychological functions vary in periodicity. Infradian and ultradian rhythms are biological patterns that vary in frequency and duration, affecting bodily chemicals and functions. Multiple instances of ultradian rhythms happen daily, while ultradian rhythms occur once every 24 hours. Infradian cycles, exemplified by the menstrual cycle, extend over weeks to reach completion. These rhythms manifest at various stages, spanning from cellular functions to social interactions, and are essential for maintaining physiological and psychological functions [10].

Circadian rhythms (CRs) are biological processes that operate on a 24-hour cycle, including chemical, physiological, and behavioral rhythms [11]. They are produced by an endogenous pacemaker or the biological timekeeper within the body or internal body rhythm, and they are influenced by environmental and external stimuli. The primary internal biological clock is located in the hypothalamic suprachiasmatic nucleus (SCN), which works closely with the pineal gland to regulate the sleep-wake cycle. The brain and other body cells include peripheral clocks, also known as circadian oscillators, which are genetically preprogrammed to produce CRs and are synchronized by the SCN. Endogenous CRs fluctuate in humans with significant intra-individual variability due to their periodic fluctuation in duration [11].

Humans, like many other photoperiodic creatures, have an intricate system for determining day and night duration, which is essential for the coordinated manifestation of physiological functions including body heat and cortisol secretion [12]. The notion of a conduit between the pineal gland and the human eye was first proposed by Descartes in 1662, which sparked interest in the process. Evidence of a multi-synaptic route linking the pineal gland and the SCN of the hypothalamus supports this 17th-century theory [12-14] and demonstrates how the SCN is essential for controlling several endocrine, physiological, and behavioral CRs [15]. The paraventricular nucleus in the retinal ganglion cells detects natural or artificial light signals, which are then sent to the spinal cord. After that, this data is sent to the thoracic spinal cord’s intermediolateral column, where second-order sympathetic fibers and primary sympathetic neurons, respectively, project to the pineal gland and superior cervical ganglion. Norepinephrine, which is released by these nerve fibers, synapses with the surface of pinealocytes to convert serotonin into melatonin, which promotes sleep. Sleep maintenance depends on this mechanism [12,16].

Chronotype should be defined as both a behavioral preference and a biological trait [11,17-20]. As a behavioral preference, chronotype refers to an individual’s preferred timing of sleep and activity, ranging from morning types to evening types. As a biological trait, chronotype is related to the endogenous circadian rhythm, influenced by factors such as clock genes, cortisol, and melatonin levels [21,22].

The MEQ, developed by Horne and Ostberg in 1976 [53], was the first validated tool to measure an individual’s morningness–eveningness tendencies or phase preference. This questionnaire classifies individuals into one of three chronotypes: 1) morning type (Larks): people who prefer early bedtimes and rise times, with a tendency to schedule activities earlier in the day; 2) evening type (Owls): people who prefer to sleep and wake later, aligning their activities with later hours; and 3) intermediate type (none of the type): individuals who show flexibility and do not strongly align with either Larks or Owls preferences.

The MEQ includes 14 multiple-choice questions and five open-ended questions that use a Likert scale format to gauge preferences (e.g., “What time would you get up if you were entirely free to plan your day?”). The questions focus on preferred timings for sleep-wake cycles, as well as mental and physical activities and levels of alertness.

Scores on the MEQ range from 16 to 86, where: 16–41 indicates an evening preference, 59–86 reflects a morning preference, and 42–58 represents an intermediate type (neither morning nor evening oriented).

In the initial validation of the MEQ [19,54], conducted with a student sample aged 18–32, the results showed that body temperature peaked significantly earlier in morning types compared to evening types, while intermediate types displayed peak temperatures between these groups. In this sample, 62.1% were classified as morning types, waking an average of 114 minutes earlier than evening types, 36.6% were intermediate types, and only 2.2% were evening types, who went to bed 99 minutes later than morning types.

Later, Taillard et al. [55] suggested updated MEQ cut-off scores based on a study of middle-aged French workers (n=566). They found that a bedtime of 11:30 PM, which might indicate morningness in students, could instead suggest eveningness in adults aged 40–50. They proposed the following MEQ cut-offs: scores 16–53, evening preference; scores 64–86, morning preference; and scores 54–63, no preference (intermediate type).

Using these criteria, they classified 20.2% as evening types, 28.15% as morning types, and 51.7% as intermediate types within their sample.

Studies across various countries consistently report the MEQ as a reliable measure, with internal consistency coefficients between 0.77 and 0.86 [14,27,56,57]. Additionally, it shows strong split-half reliability (0.80) [27] and high test-retest reliability, with coefficient ranges of 0.80–0.95 [51,58].

Several studies incorporated measurable circadian phase indicators, such as body temperature [59], as well as levels of melatonin and cortisol [24,60]. These physiological markers have been found to align well with MEQ scores, indicating that individuals’ reported chronotype preferences correspond to biological indicators of their circadian rhythms. “The MEQ itself has shown high internal consistency, with a Cronbach’s alpha of 0.83” [26]. Additionally, studies report medium-to-large correlations between MEQ scores and these circadian phase markers, supporting the MEQ’s validity in capturing actual physiological patterns of morningness and eveningness [61].

Adan and Almirall [62] developed a shortened version of the original 19-item MEQ, called the Reduced MEQ (rMEQ), which consists of five key self-report items. The first three items prompt individuals to indicate the time of day when they: 1) feel their best, 2) prefer to wake up, and 3) prefer to go to bed. The fourth item assesses the level of tiredness experienced within the first 30 minutes after waking, while the fifth item asks about overall morningness and eveningness preferences.

The rMEQ has proven to be a fast and reliable tool, showing good convergent validity with other measures of chronotype [56]. However, it has relatively low inter-item correlations, with Cronbach’s alpha ranging from 0.08 to 0.46, indicating weaker internal consistency among its items [63].

The Composite Scale of Morningness (CSM), introduced by Smith et al. [64], is a widely used 13-item self-report instrument designed to evaluate an individual’s preferences for various activities, including sleep-wake patterns. They developed the CSM by selecting items through factor analysis from the MEQ by Horne and Ostberg [53] and the Circadian Type Questionnaire by Folkard et al. [65]. It is noteworthy that nine of the items in the CSM are adapted from the MEQ.

The CSM scores range from 13 to 55, with lower scores (≤22) indicating an evening preference, higher scores (≥44) indicating a morning preference, and scores between 23 and 43 indicating an intermediate type. The scale has demonstrated strong reliability [66] with high internal consistency (α=0.87) and psychometric properties comparable to those of the MEQ and the Diurnal Type Scale (DTS). However, the original factor structure proposed by Smith and colleagues [64,67] was not fully replicated in a later study [64], with subsequent research suggesting one-, two-, or three-factor solutions [66,68-70].

Kim and Lee [71] validated the 6-item Evening Chronotype Scale (ECS), a modified version of CSM, by reverse coding six CSM items to assess eveningness. The study evaluated its psychometric properties, validity, and reliability in 472 mood disorder patients (major depressive disorder, bipolar I, and bipolar II). The ECS showed moderate to good internal consistency (Cronbach’s alpha=0.727) and external validity similar to the full 13-item CSM. Evening-oriented individuals had later sleep-wake times, longer sleep latency, more depressive/hypomanic symptoms, lower quality of life, and higher impulsivity. The study highlights the importance of assessing circadian preferences in clinical settings and supports the ECS as a reliable and valid tool for measuring eveningness.

The MCTQ, developed by Roennerberg et al. [32], is a self-report tool that differentiates between individuals’ sleep and wake times on both workdays and free days, allowing a detailed view of these patterns. This distinction between work and free days is one of the MCTQ’s defining features. Chronotype is assessed by calculating the midpoint between sleep onset and offset, adjusted to account for oversleep accumulated during the workweek [72].

According to Roenneberg et al. [73], most individuals, except early chronotypes, display significant differences in sleep timing between work and free days, typically accumulating a sleep deficit on workdays. The MCTQ, therefore, provides a quantitative measure of chronotype based on sleep behaviors rather than preferences and gives population-specific distributions for early and late chronotypes.

MCTQ scores show meaningful correlations with chemical markers such as melatonin [74] and cortisol, as well as behavioral measures like actigraphy [35] and sleep log [75]. Additional versions of the MCTQ have been developed, including the MCTQ Core [72,76] and MCTQ Shift Work [31], which include extra items, such as those related to substance use.

Many self-report questionnaires assessing chronotype are well-researched, reliable, and widely used. Instruments like the MEQ and the MCTQ are even considered gold standards in chronotype assessment due to their reliability and validity. However, they still have limitations, which will be discussed further.

The CSM, for instance, derives about two-thirds of its items from the MEQ, potentially inheriting some of its limitations. One specific issue with the MEQ is the inconsistency in scoring across studies. Horne and Ostberg [53] did not provide a clear explanation for certain item weights, such as scoring item 11 as 6, 4, 2, 0, while item 12, which asks about tiredness at 11 PM, uses a different scale: 0, 2, 3, 5 [56]. This inconsistency raises concerns about the validity and reliability of the scoring system across different studies.

Chronotype is a concept that has been understood and defined in various ways, leading to differing views on how it should be measured. “Chronotype generally refers to an individual’s preference for rest and activity within a 24-hour cycle” [11], but this broad definition encompasses a wide range of biological and psychological processes. Consequently, different researchers have chosen to focus on different aspects of chronotype to suit their conceptualization.

For instance, Horne and Ostberg [53] conceptualized chronotype as a “psychological construct,” focusing on the individual’s subjective preferences for sleep-wake patterns and activity levels. On the other hand, Levandovski et al. [50] defined chronotype as an “attribute” of an individual that reflects their sleep-wake cycle phase, emphasizing a biological perspective.

Roenneberg et al. [86] took this biological view further, proposing that chronotype should be seen as a “biological construct” aligned with the idea of “temporal behavior” or “temporal phenotype” [87], highlighting its inherent connection to an individual’s circadian rhythms. Other perspectives describe chronotype as a “dichotomous human trait” [72], or a “behavioral manifestation” and “inherited trait,” underscoring its potential genetic basis and its role as a trait that influences behavioral patterns.

These varying perspectives demonstrate that the construct of chronotype is not only complex but also multifaceted, with researchers from different disciplines interpreting and measuring it in ways that best suit their specific research aims.

The concept of chronotype is indeed multifaceted, and this complexity is reflected in the various models and definitions proposed by researchers. These differing definitions create challenges, particularly when it comes to measuring chronotype, as the measures used may not fully capture its diverse aspects.

For example, as previously mentioned, the MEQ focuses on psychological preferences, specifically assessing an individual’s diurnal preference or the preferred timing for activities like sleep and wakefulness. In contrast, the MCTQ emphasizes actual sleep timings, categorizing individuals into morning, evening, or intermediate chronotypes based on when they sleep on both work and free days.

Many other self-report questionnaires, such as the rMEQ, the CSM (circadian type scale), and MCTQ Core, similarly focus primarily on the sleep dimension of chronotype, often neglecting other important factors. These measures primarily assess sleep patterns without considering physiological markers (such as body temperature, melatonin, or cortisol rhythms) that could offer a more comprehensive understanding of an individual’s chronotype. Furthermore, these tools may overlook external influences, such as social habits, work schedules, and geographical or cultural differences, which can also play a significant role in shaping an individual’s circadian rhythm.

Hence, the reliance on these single-dimensional measures can lead to a mismatch between an individual’s reported chronotype (based on preferences or self-reported sleep data) and their actual biological chronotype (as indicated by physiological markers or behavior). This discrepancy further complicates the accuracy and comprehensiveness of chronotype assessments, highlighting the need for more integrated and multidimensional approaches that consider both subjective and objective factors influencing circadian rhythms.

The growing body of evidence highlights that “chronotype” not only varies between individuals, but it is also significantly shaped by a variety of environmental, social, and personal factors. This is essential to recognize the complexity of these factors—such as way of life, geographic context, personality features, use of drugs, work routines (freelancing, shift work, regular work, remote work), eating habits, and weight issues—when assessing an individual’s chronotype. Understanding these influences is crucial for developing a more accurate representation of chronotype, beyond sleep preferences alone.

Existing chronotype models, self-assessments, and evidence-based research have significantly deepened our comprehension of the “role of chronotype,” particularly in the context of disrupted CRs and their consequences. However, much of the existing chronotype research has been based on a somewhat simplistic view, primarily focusing on sleep-related aspects of chronotype. This narrow focus may limit the depth of insight into how chronotype influences broader health and well-being.

Building on these insights, it is essential to optimize the chronotype construct through a multifaceted model. In light of these limitations, we propose a more comprehensive, multidimensional model of chronotype, which incorporates a wider range of factors. This model would take into account not only sleep timing and preferences but also variables such as age, health, and other socio-cultural and individual factors. By expanding our understanding of chronotype in this way, future research can better address the complexities of chronotype and its disruptions, offering more tailored and effective strategies for improving health and performance outcomes across different populations.