Chronotype and Adolescence: Why being an "evening person" as a teenager is disadvantageous
Have you ever wondered how certain people in your life are able to wake up at 6am and have an incredibly productive morning such that by the time you wake up around 10am or 11am, that person has cleaned their house, walked their dog, gone grocery shopping, and written a paper? Or perhaps you are that person who wakes up at 6am and wonders how on earth some of your friends are able to stay up until 2am when you are unable to keep your eyes open past 10:30pm or 11pm. This phenomenon is scientifically known as chronotype (also termed “circadian preference”). Research suggests that individuals differ in their preferred timing of daily functioning wherein morning-types prefer to perform their daily activities in the morning or mid-day, while evening-types tend to be more active at night. Individuals with an evening chronotype typically report later bedtimes and wake times compared to individuals with a morning chronotype who report earlier bedtimes and wake times (1-4) As research has continued to unpack the role of chronotype in sleep and wake patterns, it has become apparent that chronotype is related to more than just diurnal preferences. Several studies have revealed that chronotype is also associated with personality traits (2), health behaviors (5), mental health (6), risk-taking (2,7,8), and academic achievement (9), particularly during adolescence. Notably, the data shows that individuals with evening chronotypes score lower on these outcomes when compared to their morning chronotype counterparts.
Given that adolescence is a period of high sleep variability and poor sleep quality, adolescents have been examined extensively in the sleep and chronotype literature. Adolescence is characterized by a period of critical development and increased sensitivity to social, emotional and biological cues. Given the extensive amount of change that occurs during this phase it is no wonder why adolescents tend to report poor sleep duration and quality (10) Studies have shown that pubertal development and aging during adolescence are both associated with an evening chronotype (11). While vast individual differences exist, there appears to be delay in the circadian rhythm that coincides with the onset of puberty whereby adolescents tend to sleep later and wake up later. Researchers have posited several explanations for this shift toward an evening chronotype during adolescence such as increased academic demands, social pressures and biological changes that disrupt the sleep-wake cycle during this phase of life. For example, it seems reasonable that teenagers involved in afterschool activities may need to shift their sleep cycle to allow for time to finish their homework or spend time with friends. Unfortunately, research suggests that eveningness is associated with poorer outcomes in a number of different measures. What does this mean for adolescents?
A study conducted by a group of researchers at the University of Pittsburg examined the role of chronotype on neural responses to reward during an fMRI task measuring reward and reward anticipation in older adolescents (12). Behavioral results demonstrated that adolescents with an evening chronotype reported higher rates of substance use and greater alcohol dependency. These individuals also had worse overall sleep quality compared to their morning-type counterparts. Additionally, there was a difference in neural response to reward and reward anticipation between morning-types and evening-types. Individuals with an evening chronotype demonstrated greater ventral striatum (a region implicated in the reward system) activity to reward, whereas morning chronotypes showed a response in the medial prefrontal cortex (a region associated with cognitive control) to reward anticipation. Broadly, these results suggest that evening types may have exaggerated neural reward sensitivity, which explains the increased risky behavior with regard to substance use in these older adolescents.
The prior study demonstrates the consequences associated with an evening chronotype and how those consequences map out in the brain. Other studies have found that eveningness is associated with increased impulsivity (13), lower school achievement (9), lower self-regulation (14), less positive affect (15), depressive symptoms (16), higher trait anxiety (15,16) and overall poorer sleep outcomes. Unfortunately, the previous list was not exhaustive of the negative outcomes that have been found to be associated with evening chronotypes in adolescents, thus begging the question of how to ameliorate these negative outcomes in evening-types. These results have several implications for public policy, the most important one being school start times. A U.S. Department of Education report on average school start times found that among a sample of 18,360 high schools in the United States, the average school start time was 7:59am (17). One can imagine the difficulty evening-type adolescents face when their sleep is restricted by such an early wake time. It is clear that current school timings for high school students are only conducive to the diurnal preferences of morning-types, leaving evening-types to suffer copious adverse outcomes. The findings summarized above point to the importance of accounting for chronotype in policies that are pertinent to the sleep-wake cycle.
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17. U.S. Department of Education, National Center for Education Statistics, Schools and Staffing Survey, public school data file, 2011–12. Additional information available at http://nces.ed.gov/surveys/sass/overview.asp.