Sleep is a crucial factor in maintaining both physical health and mental stability. Sleep issues are widespread among college students as well [1,2]. According to a recent large-scale study of over 7,000 American college students, the average sleep duration for American college students is 6.85 hours, and only one-third sleep more than 7 hours each night, which falls short of the recommended 7-9 hours of sleep [1,3,4]. The average sleep duration of Korean college students is even less, reported to be 6.43 hours [5]. Sleep deprivation has detrimental effects on physical [6-8] and mental health [9,10], and daytime sleepiness and fatigue can lower students’ academic performance [11-13].

Circadian rhythms refer to the physical, mental, and behavioral changes that occur on a 24-hour cycle. This natural process is primarily influenced by the presence of light and darkness in the environment [14,15]. Although often equated with sleep patterns, circadian rhythms are not synonymous with them. Circadian rhythms result from the long-term accumulation of consistent sleep patterns and light exposure over a period of more than 2 weeks. Both sleep patterns and circadian rhythms can be associated with the exacerbation of mood disorders [16-18]. However, recent research has shown that circadian rhythms, rather than sleep patterns themselves, have a more significant impact on current mood symptoms [19].

Recent studies indicate a significant increase in the incidence of psychiatric disorders such as depression among college students. A study found that during the COVID-19 pandemic, the prevalence of post-traumatic stress disorder (PTSD), depression, and anxiety significantly increased among college students. In a study involving 6,898 college students, 32.2% experienced depression, 32.1% experienced anxiety, and 15.5% experienced PTSD. The study attributed the exacerbation of these mental health issues to pandemic-induced stress and rumination [20].

According to recent research, 40.16% of 2,318 college students reported major depressive disorder (MDD) after 1.5 years of pandemic-related social restrictions, and 72.52% showed low well-being indices. Nine months after the restrictions ended, depression decreased to 28.50%, and the low well-being index improved to 53.96%. However, the prevalence of depression and issues with quality of life remain severe [21].

Attributing the increase in depression during the COVID-19 pandemic solely to stress and social restrictions may be insufficient. The social isolation imposed during the pandemic limited individuals’ outdoor activities and disrupted daily routines, potentially causing disturbances in circadian rhythms that may also be linked to the rise in depression [22].

Understanding the connection between circadian rhythms and mental health has become increasingly important as disruptions in these rhythms have been linked to various psychiatric disorders. Undergraduate students face unique challenges that make them particularly vulnerable to these disruptions, including academic pressures, lifestyle choices, and extensive use of electronic devices. Addressing these disruptions is essential for improving mental health outcomes in this population.

Understanding the connection between circadian rhythms and mental health has become increasingly important as disruptions in these rhythms have been linked to various psychiatric disorders. Undergraduate students face unique challenges that make them particularly vulnerable to these disruptions, including academic pressures, lifestyle choices, and extensive use of electronic devices. Addressing these disruptions is essential for improving mental health outcomes in this population.

Circadian rhythms are intrinsic time-keeping mechanisms that align physiological processes with the external environment’s 24-hour light-dark cycle. These rhythms are present in almost all living organisms, from cyanobacteria to humans, and are essential for maintaining homeostasis. The central circadian clock resides in the suprachiasmatic nucleus (SCN) of the hypothalamus, which coordinates peripheral clocks located in various tissues. The SCN, a small group of neurons located in the anterior hypothalamus, is the principal pacemaker of circadian rhythms in mammals. It receives direct input from the retina through the retinohypothalamic tract, allowing it to synchronize with the external light-dark cycle [23]. The SCN orchestrates daily rhythms in behavior and physiology by sending timing signals to various brain regions and peripheral organs.

The SCN communicates timing information through neural and hormonal pathways. Neural projections from the SCN innervate other hypothalamic nuclei, influencing sleep-wake cycles, feeding behavior, and thermoregulation. Additionally, the SCN regulates the release of hormones such as melatonin from the pineal gland, which signals nighttime and promotes sleep [24].

At the heart of the circadian system are core clock genes and their protein products, which form transcriptional-translational feedback loops. The primary components of this loop include Clock, Bmal1, Period (Per), and Cryptochrome (Cry) genes [25]. During the day, CLOCK and BMAL1 proteins dimerize and bind to E-box elements in the promoters of Per and Cry genes, activating their transcription. As PER and CRY proteins accumulate, they form complexes that translocate into the nucleus and inhibit their own transcription by interfering with the CLOCK-BMAL1 complex, thus completing the feedback loop.

Post-translational modifications of clock proteins, such as phosphorylation, ubiquitination, and acetylation, play crucial roles in regulating the stability, localization, and activity of these proteins. For example, the phosphorylation of PER proteins by casein kinase 1ε/δ targets them for degradation, influencing the timing of the feedback loop [26]. Similarly, acetylation of BMAL1 by CLOCK regulates its transcriptional activity and stability [27].

Peripheral clocks exist in nearly all tissues and organs, including the liver, heart, lungs, and adipose tissue. These clocks maintain local circadian rhythms and are regulated by the central clock in the SCN but can also respond to local cues such as feeding, physical activities, and temperature [28]. Peripheral clocks are essential for the temporal coordination of tissuespecific functions, such as metabolic processes in the liver and insulin secretion in the pancreas.

The synchronization between the SCN and peripheral clocks is achieved through various signaling mechanisms, including hormonal signals (e.g., glucocorticoids), body temperature cycles, and feeding schedules [29]. Disruption in the coordination between the SCN and peripheral clocks can lead to circadian misalignment, contributing to metabolic disorders, cardiovascular diseases, and other health issues [30].

Environmental factors, known as zeitgebers (time-givers), play a crucial role in entraining circadian rhythms. The primary zeitgeber is the light-dark cycle, which resets the SCN daily. Other zeitgebers include feeding times, social interactions, and temperature [31]. Disruption of these environmental cues, such as exposure to artificial light at night or irregular eating schedules, can lead to circadian rhythm disturbances.

Circadian rhythms, the natural 24-hour cycles governing physiological and behavioral processes, are crucial for maintaining mental health. Disruptions to these rhythms have been linked to a range of psychiatric disorders, underscoring their importance in regulating mental stability. Research indicates that a misalignment between an individual’s internal clock and external environmental cues can precipitate mood disorders such as depression and anxiety [32]. For instance, irregular sleepwake cycles and insufficient sleep are commonly associated with heightened levels of stress and increased depressive symptoms [33]. This is particularly concerning given the high prevalence of these irregular sleep patterns in modern society.

Chronic stress and an irregular lifestyle can exacerbate mental health issues, including insomnia, depression, and anxiety. When the circadian rhythm is disrupted due to irregular sleep patterns or excessive stress, it can lead to prolonged psychiatric symptoms (Figure 1). The body’s inability to synchronize the sleep-wake cycle with environmental cues results in cognitive impairment and increased susceptibility to mental health disorders [32].

We demonstrated that individuals with a predisposition to circadian rhythm disruptions exhibited a significantly higher likelihood of experiencing mood disorders. This study emphasized that the stability of circadian rhythms is not merely a background factor but a critical component in the maintenance of mental health [34]. We conducted a comprehensive study involving 31 mood episodes in 26 patients with bipolar disorder (BD) and 18 controls. We evaluated the circadian rhythms of these individuals at admission, at 2-week intervals during hospitalization, and at discharge. The study revealed distinct circadian phase shifts during acute mood episodes compared to recovered states. In 23 acute manic episodes, circadian phases were advanced by approximately 7 hours. After treatment, phases in 21 of these cases returned to normal with a delay of around 7 hours, indicating a recovery of the normal circadian phase [34].

Moreover, the study found that during mixed manic episodes, circadian rhythms were delayed by approximately 6-7 hours, while depressive episodes exhibited delays of around 4-5 hours [34]. These findings highlight the critical role of circadian misalignment in the pathophysiology of BD.

Delayed circadian rhythms, characterized by later sleep and wake times, are particularly prevalent among college students. This demographic often exhibits a preference for evening activities and late-night studying, resulting in sleep phase delays. Such patterns are not merely lifestyle choices but are influenced by biological changes during adolescence and early adulthood. Crowley et al. [35] note that delayed sleep phase syndrome (DSPS) is common in this age group, affecting up to 7%-16% of young adults. This delay can lead to chronic sleep deprivation due to misalignment with academic and social schedules, further exacerbating mental health issues [36]. The cumulative effect of insufficient and irregular sleep contributes to impaired cognitive function, reduced academic performance, and increased susceptibility to psychiatric disorders. Disruptions in circadian rhythms have been associated with mood disorders, particularly depression and BD [16].

Circadian rhythms play a vital role in the mental health of college students, with disruptions linked to various psychiatric problems. The prevalence of delayed circadian rhythms in this age group, compounded by the pervasive use of smartphones, underscores the urgency of addressing sleep hygiene and circadian alignment to mitigate mental health risks. Future research and mental health interventions should focus on promoting regular sleep patterns and reducing nighttime exposure to electronic devices to improve overall well-being in college students.

Circadian rhythms relate to multiple aspects of health and well-being, including physical activity patterns. Susceptibility to circadian regulation predisposes individuals to circadian misalignment, poor sleep, sleep deprivation, increased sleepiness, and consequently, sedentary behavior.

Biological changes during adolescence and young adulthood contribute significantly to delayed circadian rhythms, marked by a preference for later sleep and wake times. Sleep pressure decreases during adolescence, resulting in delayed sleep schedules [37,38]. Weekday and weekend sleep patterns vary, with more catch-up sleep on weekends [39]. Adolescents’ circadian regulation is particularly vulnerable, which may lead to sedentary behavior [40].

The two-process model of sleep regulation, involving homeostatic sleep pressure and the circadian pacemaker, is disrupted during adolescence [41]. Sex-specific differences emerge, with adolescent girls experiencing longer sleep onset latencies and more insomnia symptoms, while boys exhibit less regular sleep patterns and later bedtimes [42].

Delayed sleep phase disorder (DSPD) is common in adolescence, characterized by a delayed sleep schedule and daytime impairment [43]. DSPD desynchronizes patients from societal schedules, leading to sleep loss and daytime sleepiness [41]. Prevalence estimates range from 3.3% to 16%, with higher rates in girls [43,44]. Delayed sleep phase is linked to mental health issues like depression and anxiety [45,46].

The demanding academic and social schedules of college students also contribute to delayed circadian rhythms. Balancing classes, homework, jobs, and social activities leads to irregular sleep patterns. Late-night studying or socializing delays sleep onset and disrupts circadian rhythms [36]. Academic pressure increases stress, exacerbating sleep disturbances and circadian misalignment [47].

Late-night social media use also mediates the association between diurnal type and sleep quality. Watching movies or listening to music late at night was the strongest mediator of the association between diurnal type and sleep and tiredness. The most prominent finding shows that of all different media forms, watching movies or listening to music late at night were associated with increased daytime tiredness, whereas late social media use was associated with poor sleep quality [48]. The proliferation of smartphones has exacerbated the disruption of circadian rhythms among college students. Exposure to blue light emitted by screens can suppress melatonin production, a hormone crucial for sleep regulation, thus delaying sleep onset [49]. The widespread use of smartphones for social media, gaming, and studying late into the night has made it increasingly difficult for students to maintain regular sleep patterns. A study by Lemola et al. [50] found a strong correlation between high smartphone usage at night and increased levels of sleep disturbances and depressive symptoms. This digital interference in natural sleep cycles is a significant contributor to the growing mental health crisis in college populations, highlighting the need for interventions aimed at reducing screen time before bed.

Lifestyle choices, including diet, exercise, and substance use, also influence circadian rhythms. Irregular eating patterns, lack of physical activity, and caffeine consumption disrupt the sleep-wake cycle. Caffeine use for late-night studying interferes with regular sleep [51]. Irregular meal times and lack of natural light exposure during the day, especially in the morning, further disrupt circadian rhythms [14].

Maintaining a healthy circadian rhythm is crucial for college students as it significantly influences their overall health, academic performance, and well-being. Here are key aspects to consider (Table 1).

Early adulthood is a critical period often associated with the onset of various psychiatric disorders. MDD is one of the most common mental health disorders. In the general population, the lifetime prevalence of MDD is about 15%-20% [71,72]. However, among individuals aged 18-25, the prevalence is notably higher, with approximately 18.6% experiencing a major depressive episode [72]. This increased incidence is likely due to the significant life changes and stressors characteristic of early adulthood. Anxiety disorders are also prevalent during early adulthood. The lifetime prevalence of anxiety disorders in the general population is around 25%-30% [71]. In the 18-29 age group, the prevalence ranges from 20%-30%.

BD has a general population prevalence of about 1%-3% over a lifetime [73]. However, the first episode of BD typically occurs between the ages of 18 and 25, making early adulthood a high-risk period for the onset of this condition. Schizophrenia is another severe psychiatric disorder that often emerges in early adulthood. Its lifetime prevalence in the general population is about 0.5%-1% [74]. The typical age of onset is in the early to mid-20s for men and late 20s for women, placing early adulthood as the critical period for the first presentation of symptoms.

Eating disorders, including anorexia nervosa and bulimia nervosa, show a higher prevalence among young women aged 18-25. In the general population, the lifetime prevalence of anorexia nervosa is approximately 0.5%-1%, and bulimia nervosa is about 1%-3% [75]. In early adulthood, the prevalence can increase to around 1%-4%, particularly among females, due to heightened concerns about body image and societal pressures. Substance use disorders are also more common during early adulthood. The lifetime prevalence of alcohol use disorder is approximately 12%-17%, and drug use disorders are about 8%-10% in the general population [76]. For individuals aged 18-25, the prevalence of substance use disorders can be as high as 20%-25%.

Early adulthood mental illness treatment often involves a combination of psychotherapy, medication, and lifestyle modifications to effectively manage symptoms and improve overall well-being. However, as previously mentioned, this age period is characterized by disruptions in the circadian rhythm, and it is common for delays or disturbances in circadian rhythm to occur at this age. Therefore, it is crucial to additionally apply therapeutic approaches that help correct the circadian rhythm.

Maintaining stable circadian rhythms is essential for mental health, particularly among undergraduate students. Modern lifestyle factors, including the pervasive use of electronic devices and the implications of social distancing measures, significantly challenge circadian rhythm stability. However, with appropriate interventions, such as lifestyle modifications and the utilization of technological tools, it is feasible to mitigate these effects and enhance mental health outcomes. Continued research and the implementation of circadian rhythm-focused strategies in mental health care are imperative. Resetting an already delayed circadian rhythm necessitates a multifaceted approach that combines light therapy, chronotherapy, melatonin supplementation, cognitive-behavioral strategies, and lifestyle adjustments. It is crucial for individuals to collaborate closely with healthcare providers to develop a personalized treatment plan tailored to their specific needs and circumstances. With consistency and adherence to these methods, it is possible to realign the circadian rhythm and improve overall sleep quality and daily functioning.