Is a later school start time better for teenagers?
“Yes, of course”, you say! But how do you know that?
What if I told you that the correct answer is “It depends”?
Does School Start Time Matter?
NIMH investigators asked questions about bedtimes, amount of sleep, and various other questions to tease out important demographic information. Weeknight bedtime was assessed with the question “What time do you usually go to bed on weeknights?” Sleep duration was assessed with the question “How many hours of sleep do you usually get on week (weekend) nights?” They asked this question to indicate whether the teenagers slept at least 8.5 hours of sleep. This number was chosen, instead of 9.5 hours, as is currently recommended, because at the time of the study, the National Sleep Foundation (NSF) recommended only 8.5 hours for teens.
Any high school kid will tell you that it’s much more complicated to understand the truth about their sleep. For this reason, the NIMH investigators asked about family characteristics, after-school jobs and extracurriculars. They also asked about school type (public vs. private, etc) and grade level.
Here’s What They Found
The NIMH school start time study is a good example of why it’s never a good idea to assume you know the answer to a question. Always investigate.
Need a SLEEP COACH?
The average number of weeknight sleep hours was 7.71 hours (7.60 for girls and 7.81 for boys). Not only was this result almost an hour less than the recommended NSF nightly allowance, it was almost two hours less than current recommendations!
But as this number was an average, it is obvious that some teenagers slept less than 7.71 hours, and some slept more. Extra sleep was associated with some surprising findings, which we’ll get to soon.
When school started later, students went to bed later. This was one of the expected findings. And here is where it gets interesting.
School Start Time Matters… Until 8 AM
The investigators looked at school start times and then asked how many students got adequate sleep at each starting hour (e.g., 6:30 AM, 7:00 AM, etc.) When school started later, students got more sleep and more got the recommended amount of sleep (8.5 hours), but the effect went away for start times after 8:00 AM. In other words, if school started at 8:30 AM, or 9:00 AM, the students on average did not get more sleep! There appeared to be “diminishing returns”. In other words, later school start times was good for high schooler’s sleep… up to a point, namely 8:00 AM.
It Gets More Complicated…
It turns out that the benefits of later school start times may fall to boys only. When investigators looked at boys and girls separately, they found some unexpected results. School start time was not associated with hours of sleep for girls, regardless of start time. Similarly, later start times did not correlate with girls getting their 8.5 hours of sleep. School start time just didn’t seem to have anything to do at all with girl’s sleep.
What about the boys? Since girls’ sleep didn’t seem to be affected by school start time, you would think that when you removed girls from the analysis that the effect would be even bigger for boys.
And it was… up to a point.
When school started later, boys got more sleep, and more boys got “adequate sleep” but the effect went away after 8AM.
City Boys vs. Country Boys
It turns out that location of the school mattered quite a bit. Investigators divided the kids into three groups: major metropolitan county (census-defined metropolitan counties with ≥ 1 million residents), other urbanized county (metropolitan counties with < 1 million residents), and non-urban county (non-metropolitan counties).
For boys living in major metropolitan areas, later start time was associated with adequate sleep, but only until 8 AM. For boys living in “other urbanized counties” and non-urban counties, start time had no effect on adequate sleep. Put another way, for boys going to big-city schools, later start times were associated with better sleep (up until 8 AM, of course…) For boys in smaller cities or suburbs, start time appeared to have no effect on quality sleep.
Once again, girls sleep was unaffected by location. City girls got just as much sleep as country girls.
In an earlier post, we showed that teenagers tend to sleep differently on weekends. These are the days we expect teenagers to “make-up” on sleep.
It is reasonable to guess that the teens who go to high schools with later start times might not need to make up for lost sleep on weekends. Indeed this is what the NIMH investigators found at first… until they “adjusted” the results. “Adjustment” means eliminating or reducing the confounding effects of extraneous confounding factors like sex, age, etc. The NIMH investigators adjusted data for all their results in this study, but when they used adjustment to look at make-up sleep, they got an unexpected result:
There seemed to be a correlation between later start times and less make-up sleep, but the effect would go away after normalizing for age, sex, school level, and school location (city vs. country, etc). In other words, overall later school start time did not give teenagers the opportunity to make up for lost sleep.
Strengths of the Study
Large studies are always better than small studies. The statistics always get better for one thing. Put another way, you can resolve small differences, and bring out subtleties better if your study has more subjects. We believe something closer to the truth is found when you look at very large populations. Almost 10,000 high school kids is a large enough sample size to give results we can believe. And indeed this study found some interesting differences: between boys and girls, and between city boys and country boys.
The large sample size also allowed the NIMH investigators to uncover the compelling finding about the hour of 8:00 AM: Later start times did indeed matter, but the effect would disappear after 8.
Limitations of the Study
Survey results are never the best way to arrive at the truth. Think of those times in American history when opinion polls grossly mis-calculate the outcomes of elections! The NIMH school start time study was limited by use of a cross-sectional survey as the research tool. As a result, all the investigators could tell us is that there were associations between things like school start times and hours of sleep. They were unable to tell us if any two things were causally related to each other.
Furthermore, it’s far more accurate to measure hours of sleep than it is to ask a teenager how long she slept! In a study this large, actual measurement was not possible. They also did not ask the kids about confounders like cell phone use, which may vary from group to group.
Finally, there is the question of generalizability. It is important, when reading about studies like this on the internet, to ask an important question: “Does this study apply to me?” If this same study were performed on teenagers living in isolated villages deep in the Amazon River basin, you might justifiably say “I’m not sure this study tells me anything about my teenager’s sleep”. But you can still ask the question about studies done on American teenagers. Look carefully at the study and ask yourself how easily your child could have been one of the kids who turned in a survey.
Summary: The NIMH School Start Time Study Suggests…
- That teenagers don’t get enough sleep
- That later start times matter, but
- only if you’re a boy, and
- only if you live in a big city
- only until 8 AM
What they’re saying about Sleep, Baby!