Author: Patrick Toh
Editor: Hayley Chan
Artist: Subhasri Mazumdar
Introduction
Have you ever wondered why some people prefer to sleep early, and some prefer to stay up in the late hours before heading to bed? Scientists have an answer for this, and it is called chronotypes. In simple terms, it can be described as similar to an astrology sign or zodiac sign, but the scientific definition is an individual’s preference for sleeping and wakefulness, highly dependent on our circadian rhythm, in layman’s terms is the natural inclination of your body to sleep at a certain time according to the Sleep Foundation.
However, our social life and work/school schedule can cause a severe misalignment in our sleep schedule and internal biological clock, hitting those with the evening chronotype the hardest. This phenomenon is called social jetlag, and it can lead to many mental health issues and, in the long term, sleep disorders.
Chronotype: Biological Basis and Classification
Our chronotype is largely determined by our circadian rhythm, controlled by oscillators found on the suprachiasmatic nucleus (SCN) in the hypothalamus. The circadian rhythm is our body’s 24-hour internal clock, regulated by our genes and external factors like light and dark. It can influence many parts of our body, like sleep, activity, and appetite.
Chronotypes are classified into roughly three types: Morning (lark), evening (owl), and intermediate/neither chronotypes. Morning chronotypes prefer to wake up and sleep early and do activities early in the morning, while evening types wake up and sleep late and perform activities late in the evening/night. Finally, most of the population falls between the middle of the two extremes. Interestingly, unlike the zodiac or astrology signs, we do not have a fixed chronotype. When we are adolescents, we initially lean more towards the evening chronotype, but once we enter our early twenties, we become more morning-orientated the older we get.
Impact of chronotypes on mood disorders and mental health
Numerous chronotype studies have found a link between evening chronotype and mood dysregulation. Because of work/school schedules, individuals would have to wake up early and return home extremely late. Those with evening chronotypes will especially struggle with the misalignment of their circadian rhythm. Night shift workers will also experience extreme misalignment, particularly affecting those with morning chronotypes. This misalignment can lead to individuals adjusting their sleep schedule during the weekends compared to weekdays. This phenomenon was introduced by Wittman et al. who term it social jetlag, and it can have a profound effect on individuals if this misalignment occurs in the long term. A study looking at undergraduates found that those with evening chronotypes had a significant difference in rise time and total nocturnal sleep between weekends and school days.
Consequently, they correlated higher rates of sleep-related problems in evening types compared to the morning types, reporting more daytime sleepiness due to the misalignment between their biological circadian rhythm and school schedule. Additional factors, such as adolescence, social demands, relaxed parental restrictions, and increased independence, can contribute to the misalignment. Further studies on university students have also found that this lack of synchronisation can worsen their academic achievement and complicate their relationships with family members and school friends.
Furthermore, chronotypes have also been associated with multiple mental disorders. Those with evening chronotypes were more likely diagnosed with bipolar disorder and associated with more frequent bipolar episode recurrences. However, the literature about this connection is mixed; two recent studies did not find any risk for bipolar. Researchers have also investigated the relationship between ADHD, anxiety, and psychotic disorders with chronotypes.
Conclusion
We all know sleep is essential to our health. But research into chronotypes—our preference for sleeping and waking up—can give insights into our biological circadian rhythm and associations with mental health disorders. In turn, clinicians can help combat the growing rise in mood disorders in individuals due to the pressure of social and work demands.
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