Should high school start later?

A teenager's clock really does run late

Parents often read a teenager who won't sleep at 10 pm and won't wake at 7 am as lazy. The science says otherwise.

In 1993, Mary Carskadon and colleagues at Brown University surveyed 458 children, all in sixth grade and so all about the same age. They asked how far through puberty each child was, and whether they felt more like a morning or an evening person. They also checked school and birth order. The further through puberty, the more the child leaned towards evenings; the link was clear in girls, with the same trend in boys. School and family made no difference. The team concluded that something biological drives the shift (Carskadon et al., 1993).

Later work confirmed it: sleep and wake times move later through the second decade of life, driven partly by the body clock itself and partly by how slowly sleep pressure builds in a teenage brain (Crowley et al., 2007). A teenager lying awake at 11 pm isn't choosing to. The clock hasn't said "night" yet.

The body clock and why its timing matters are covered in How much sleep do you really need? Teenagers are the extreme case of a body clock out of step with the alarm.

Who set the bell?

The early start wasn't designed around teenagers. Before the 1960s, American schools rarely began before 8:30 or 9 am. As suburbs spread, most public high schools moved to around 8 am, and some to 7 am (Wikipedia, 2026).

The reasons are adult ones:

• Buses. One fleet runs several rounds: high school first, then middle, then primary. The oldest students get the earliest run.
• Care of younger children. An early finish lets older students collect younger brothers and sisters, which matters most in families where parents work long or fixed hours.
• Traffic. Early buses miss the 8:30 am rush.

Each reason makes sense on its own. Together they built a timetable where the students with the latest body clocks start first. Nobody chose that on purpose; the system added it up that way.

What the doctors say

In 2014 the American Academy of Pediatrics called early school start times "a key modifiable contributor to insufficient sleep" in teenagers. It recommended that middle and high schools start no earlier than 8:30 am, so students can get the 8.5 to 9.5 hours of sleep they need. It linked later starts to better physical health, less depression, less drowsy driving and better results at school (Adolescent Sleep Working Group et al., 2014).

Seattle tried it

For the 2016–17 school year, Seattle Public Schools moved its high school start from 7:50 to 8:45 am. Researchers put wrist activity monitors on Year 10 students at two high schools, Roosevelt and Franklin, for two weeks in the spring before the change and again the spring after (Dunster et al., 2018).

What they measured	After the change
Sleep on school nights	34 minutes more (median)
Wake-up time	44 minutes later (median)
Bedtime	No significant change
Grades	4.5% higher (median)

The bedtime result answers the usual worry. Students didn't just stay up later and waste the extra hour. They went to bed at about the same time and slept in.

The best news came from Franklin. In that school, 88% of students were economically disadvantaged, against 31% at Roosevelt. Only at Franklin did first-period absences and late arrivals fall significantly (Dunster et al., 2018). The students with the least room in their lives gained the most from a change that cost them nothing.

The study is small, about 90 students a year, and compares two year groups rather than following the same students. It fits a larger body of work, though, and the direction is the same.

California made it law

In October 2019 California became the first US state to set a floor. Its law, SB 328, requires high schools to start no earlier than 8:30 am and middle schools no earlier than 8:00 am, from July 2022 or when existing staff agreements ran out, whichever came later. Rural districts are exempt, and schools may still run optional early classes for a few students (California Legislature, 2019).

What about Australia?

Australian teenagers already start later than most Americans. A 2013 study compared 385 Australian and 302 US high school students using a week of sleep diaries. The Australian schools started at 8:32 am on average, the US ones at 7:45 am. Australian students slept 47 minutes more on school nights (Short et al., 2013).

The start time wasn't the only difference. Australian students were more than twice as likely to have a bedtime set by a parent (17.5% against 6.8%), and they spent about an hour less a day on sport, clubs and other commitments. All three were linked to more sleep. The picture is of a culture that leaves teenagers more time, not just a later bell.

The costs are real

A later start isn't free, and the people who pay are usually families and schools, not researchers.

• Buses. If high schools can't take the first run, a district may need more buses and drivers, or must move the primary schools earlier.
• Sport and jobs. A later finish pushes training, matches and part-time work into the evening.
• Parents' work. Care before and after school must fit hours that were set for the old timetable, and older children may no longer be home to mind younger ones.

These are problems of logistics. They are solvable: Seattle solved them. The biology isn't negotiable.

What you can do

1. Find out when your teenager's school starts. If it's before 8:30 am, ask the school why.
2. Ask what the first period is used for. Some schools put optional or study sessions there.
3. At home, treat a late-sleeping teenager as normal biology, and protect the hours that are left: dim lights and screens down in the last hour before bed.
4. Help keep weekday and weekend wake-up times within an hour or two, so the clock doesn't jet-lag every Monday.