During adolescence, significant physiological changes, such as delayed melatonin secretion and brain development, lead to alterations in circadian rhythms and sleep patterns. Consequently, adolescents exhibit a characteristic shift toward later bedtimes and wake-up times (1). Sufficient and high-quality sleep is crucial for adolescents’ physical and mental health, cognitive function, academic performance, and emotional regulation. Severe sleep issues can negatively impact both their physical and mental health (2). Sleep deprivation in adolescents is associated with declines in academic performance, increased emotional problems, and reduced cognitive function (3–10). Additionally, insufficient sleep is a risk factor for conditions such as obesity (11), hypertension (12), and cardiovascular diseases (13).

Nowadays, sleep deprivation is prevalent among adolescents. Surveys from the United States indicate that over 70% of high school students sleep less than the recommended minimum of 8 h. This proportion has continued to rise over the past decade, and the quality of sleep is also declining (14). According to the “2019 China Youth and Children’s Sleep White Paper,” which surveyed 42,339 students and their parents, the average sleep duration for adolescents aged 13 to 17 is less than 7 h. This is significantly shorter compared to previous years, and most teenagers are not getting enough sleep. In response to this issue, in March 2021, the Chinese government issued a notice to strengthen sleep management for primary and secondary school students (15). The notice emphasized the need for schools to balance work and rest times, reduce the academic workload, and include sleep status as part of physical health assessments. Later, in July 2021, the Chinese government issued another notice to further address sleep management. This was part of broader efforts to reduce the homework and extracurricular training burden on students, known as the “Double-Reduction” policy (16). The main reason behind the Chinese government’s “Double Reduction” policy is the heavy academic pressure faced by Chinese students. In response to this pressure, Chinese students often spend more time studying, either by choice or because they feel they have to. Schools sometimes extend class hours or add extra tutoring sessions. Students also attend after-school tutoring or arrive early for self-study. These practices often cut into their sleep time. To address this issue and give students more control over their schedules, the Chinese government launched the “Double Reduction” policy. These measures aim to ease academic pressure, give students more time for sleep, and support their overall physical and mental well-being.

Sleep duration, wake-up time, and bedtime are important measures for understanding sleep patterns (17). Most studies on teenage sleep rely on cross-sectional studies, that do not accurately assess continuous changes in sleep in the adolescent developmental period. Compared to the recall bias inherent in subjective questionnaires (18), objective measurement devices can accurately and objectively record relevant sleep parameters, providing new data sources and research methods for sleep studies.

The “Double Reduction” policy (July 2021) represents a major educational reform in China aimed at alleviating students’ academic burdens. Against this new policy backdrop, long-term, objective monitoring data on secondary students’ sleep patterns remains scarce. Therefore, this study aims to utilize wearable devices to initiate a three-year longitudinal investigation starting after policy implementation (December 2021), describing the evolving trajectories of secondary students’ sleep parameters.

Adolescent sleep is a critical determinant of overall health and well-being. This study thoroughly examines sleep patterns among Chinese adolescents. It aims to provide a scientific foundation for evaluating their sleep status and sleep sufficiency rates. Polysomnography (PSG) is considered the gold standard for measuring sleep. However, its operation is complex, costly, and time-consuming. These characteristics make it more suitable for small-scale comparative studies. Previous adolescent sleep studies have mostly used questionnaires, which are simple but may have recall bias. In contrast, this study used wearable technology, which can collect adolescent sleep parameters more objectively and accurately, providing reliable data support for a comprehensive understanding of adolescent sleep characteristics.

That study examined the sleep patterns of middle school students over 3 years (seventh, eighth, and ninth grades) after implementation of the policy. Findings indicate that students’ nighttime sleep duration decreased progressively with grade level, dropping from 452.3 ± 32.4 min in G7 to 420.4 ± 37.9 min in G9. Furthermore, students’ average sleep duration remained persistently below the recommended 8-h health standard (25). As students advance through the grades, the persistent decline in sleep duration is a cause for concern. The American Academy of Sleep Medicine recommends that adolescents aged 13 to 18 should regularly sleep 8 to 10 h to promote health (25). Therefore, this study classified adolescents who slept less than 8 h on average as sleep deprived. This study has shown that the prevalence of sleep deprivation among adolescents has increased over the years. This finding consistents with the results of a systematic review, which examined sleep duration among children and adolescents in 20 countries from 1905 to 2008 (26). It found that adolescents’ hours of sleep declined rapidly from year to year during this period, and the risk of developing sleep deprivation increased. Additionally, the night sleep duration of adolescents in this study was significantly lower than that of their peers in the United States (18), Malaysia (27), Australia (28), and Canada (29). The reported rate of sleep deprivation among students in each of the 3 years of middle school exceeded 80%, which was much higher than the national average (30). This means that Chinese middle school students still suffer from sleep deprivation.

The study found that over the 3 years, middle school students experienced a gradual reduction in light sleep, deep sleep, and REM sleep, while daytime sleep duration initially decreased and then increased. The significant reduction in daytime sleep during the second year may be attributed to the necessity of nucleic acid testing or other epidemic prevention measures implemented by schools during the COVID-19 pandemic, which occupied students’ nap time. This unavoidable confounding factor is associated with decreased daytime sleep duration. Research suggests that increasing nap time to make up for shorter night sleep can help students reach 8 h of total sleep, improving their cognitive function (31). Therefore, the reduction in daytime sleep for G8 students may have left them unable to compensate for sleep deprivation, resulting in issues such as daytime inattention. Additionally, the gradual decline in deep sleep duration from G7 to G8 may indicate a decrease in sleep quality. Deep sleep is considered a critical period for physical repair and recovery. This reduction reflects not only the overall decrease in total sleep duration but also a potential decline in the quality of sleep among adolescents.

In the study results, there was no significant difference in night sleep duration between males and females. However, gender had a significant effect on deep sleep duration and daytime sleep duration, with females having significantly more than males in both. Meanwhile, the interaction between time and gender on deep sleep duration was significant, and there were differences in the trends of deep sleep duration between males and females. This phenomenon among adolescents may be due to differences in physiological developmental rates and hormonal changes between genders. Male and female have different sleep patterns, including sleep quality and sleep structure. This is due to differences in thermoregulatory mechanisms and hormone levels between genders, which make biological rhythms inconsistent (32, 33). Females in this study had more hours of deep sleep and daytime sleep than boys, which is consistent with Roberts et al.’s study that women usually perform better in objective sleep indicators (34). Beyond physiological differences between genders, social factors such as gender roles, social opportunities, and lifestyle choices are hypothesized to influence sleep gender differences. However, due to the lack of explanatory variables in current research (e.g., adolescent mental health, pre-sleep procrastination, stress levels, dietary patterns, sleep hygiene, physical activity), coupled with the absence of questionnaire surveys to analyze sociocultural factors in this study, we cannot definitively elucidate the mechanisms underlying gender differences in adolescent sleep (35). This thus provides new directions for future research.

On school days, wake-up times were significantly earlier than on rest days, and sleep onset time gradually shifted later each year. This is because school schedules require students to wake up early on school days, while increasing academic pressure forces them to spend more time completing homework at night, leading to delayed sleep onset times. This schedule conflicts with adolescents’ natural sleep patterns. During adolescence, the secretion of melatonin, a hormone that regulates the sleep–wake cycle, is delayed at night compared to childhood. This delay results in later sleep onset times and a shift toward a “late to bed, late to rise” sleep pattern (36).

In this study, sleep onset time and wake-up time on rest days were later than on school days, and night sleep duration on rest days was significantly longer than on school days. This suggests that students tend to extend their sleep time on rest days to compensate for sleep deprivation during school days. This finding aligns with previous research, which conducted a cross-sectional study on the sleep habits of 735 adolescents, showing that adolescents experience insufficient sleep on school days due to academic pressure and heavy schedules, leading them to “compensate” by waking up later on rest days (37). However, this behavior may lead to more severe sleep rhythm disruption. This misalignment between biological and social clocks is defined as social jetlag (SJL), which serves as an indicator for assessing circadian rhythm disruption. Studies indicate that more severe SJL is associated with a higher risk of adverse health outcomes, including cognitive dysfunction, attention deficits, and mental disorders. The later sleep onset time on rest days may be attributed to parents allowing adolescents more leisure time, leading to increased use of electronic devices such as phones, computers, and tablets. The blue light emitted by these devices can suppress melatonin secretion, disrupt sleep patterns, and result in later sleep onset times (38). This delay in adolescents’ biological clocks conflicts with school schedule, which can lead to reduced hours of sleep at night potentially causing sleep problems and negatively affecting students’ physical and mental health (39). The study also found that daytime sleep duration was longer on school days than on rest days, which may be due to the fact that students compensate for insufficient night sleep on school days by taking naps to alleviate fatigue. When sleep deprivation occurs, the human homeostatic sleep drive increases, resulting in more fatigue and sleepiness during the day, which prompts adolescents to relieve fatigue by taking a nap (40). While napping may temporarily relieve fatigue caused by sleep deprivation, frequent napping can negatively affect nighttime sleep duration and quality, creating a vicious cycle (41).

Daytime sleep duration, wake-up time and sleep onset time showed significant interactions between time and time type (school day/ rest day). There was no statistically significant difference between the 3 years in wake-up time on rest days, but wake-up time on study days was delayed by about 6 min in the third year compared to the first year. However, the delay was only 6 min, failing to achieve a meaningful increase in sleep duration. At the same time, students continued to face substantial academic pressure and heavy homework loads, resulting in persistently delayed bedtimes on school days. Future schools should consider adjusting school schedules and incorporating education on the healthy use of smartphones. For instance, refrain from using smartphones, tablets, and other blue light-emitting devices at least 60 min before bedtime, blue light significantly suppresses melatonin secretion, disrupts circadian rhythms, and leads to difficulty falling asleep (42). It is recommended to delay school start times. Multiple studies have found that delaying the start of the school day by 30 min increases sleep duration by 11–20 min (43). This is not merely a simple addition of time; more crucially, it allows students’ sleep schedules to better align with their naturally delayed circadian rhythms, thereby improving sleep quality.

Limitations of this study. First and foremost is the inherent limitation of the research design. Due to the lack of baseline data prior to the implementation of the “Double Reduction” policy and the absence of a control group unaffected by the policy, we cannot causally attribute the observed sleep changes to the policy. Confounding factors such as the physiological circadian rhythm delay associated with adolescent development and the inherent increase in academic pressure with grade level may significantly influence sleep patterns. Therefore, the findings of this study should be interpreted as a valuable description of the sleep trajectories of middle school students during a specific historical period (post-“Double Reduction”) and within the policy context, rather than an evaluation of the policy’s effectiveness. Future studies employing quasi-experimental designs will be essential for more accurately assessing the policy’s impact.

This study reveals a trend of progressively shorter sleep duration among middle school students over 3 years, with persistent worsening of sleep deprivation issues. These findings indicate that, within the current policy context, middle school students’ sleep health continues to face significant challenges, influenced by complex factors. Recommendations include delaying morning class start times and strengthening sleep hygiene education for both parents and students to address this ongoing public health issue.