Humans evolved to be in sunlight from sunrise to sunset and it is only fairly recently that we have moved primarily indoors during the day. Studies show numerous physiological and health benefits from spending more time outdoors during the day, including:

Decreased incidence of myopia. Increasing the amount of time spent outdoors during the day reduces the incidence of myopia and slows the progression of existing myopia in children. The incidence of myopia is increasing worldwide and the current prevalence of myopia by ages 15-20 in the United States is over 50%. A 2019 meta analysis in children ages 4-14 recommended a minimum of 120 minutes of outdoor sunlight exposure at school per day to reduce the incidence of myopia.

Improved sleep/wake schedule. Increased time outdoors, especially in the morning, can help shift the sleep schedule of children, adolescents, and young adults earlier, making it easier for them to get to bed early and wake up in time for school. During puberty, the timing of sleep naturally shifts later so helping middle school and high school students to wake up earlier is especially important. 

Longer sleep duration. Increased time outdoors in children and adolescents is correlated with longer sleep durations at night. A 2019 survey by the CDC (Youth Risk Behavior Survey) found 77.0% of high schoolers in the United States get fewer than the recommended 8 hours of sleep on school nights. The consequences of chronic sleep deprivation in teenagers are severe, leading to increased motor vehicle accidents, increased mood disorders, reduced academic performance, and increased risk taking behaviors. Increased sleep durations in teenagers have been shown to result in improvements in academic performance, reduced tardiness, improved athletic performance and reduced car accidents.