This study used data on 2,700 teenagers from a large US survey called Add Health. They were surveyed in grades 7 to 12, a year later, and in 2001 to 2002, at ages 18 to 26. The teens reported their own bedtimes and usual sleep. The researchers compared teens in the latest third of bedtimes with those in the middle third. Teens with early bedtimes were left out of these comparisons. The paper does not report whether any teens had ADHD.
Teens with late school-year bedtimes tended to sleep less. At the second survey, 91.15% of them slept under 9 hours. Over time, late school-year bedtimes at either early survey were linked with lower high school grades. Late school-year bedtimes at the first survey were also linked with more emotional distress years later. Late summer bedtimes were linked with more later distress, but not with lower grades. Sleeping under 9 hours did not predict either outcome, though an extra check found that habitually sleeping under 6 hours at the first survey went with worse grades later.
The study only watched and recorded, so it cannot show that late bedtimes caused these outcomes. All the sleep data were self-reported. The authors note that learning disorders and ADHD often go with late bedtimes and may explain part of the grade gap.
The authors suggest future research should focus on school-year bedtime in teens aged 14 to 16 as a target for prevention.
The numbers Stats and what the terms mean
The statistics behind the plain-language claims above.
- Asarnow, 2013: later school-year bedtime and shorter sleep at the second survey, B = −.19; p < .001.
- Asarnow, 2013: late school-year bedtime at the first survey and later high school GPA, b = −.27, p <.001.
- Asarnow, 2013: late school-year bedtime at the second survey and later high school GPA, b = −.13, p <.01.
- Asarnow, 2013: late school-year bedtime at the first survey and later emotional distress, OR 1.35, p <.05.
- Asarnow, 2013: late summer bedtime at the second survey and later emotional distress, OR 1.4, p <.01.
- Asarnow, 2013: sleeping under 9 hours and later GPA or emotional distress, all p >.05. What the terms mean:
- B or b A number showing how much one measure changes along with another. A minus sign here means later bedtimes went with less sleep or lower grades.
- OR (odds ratio) A comparison of how likely an outcome is in one group versus another. A value above 1 means the late-bedtime group was more likely to report emotional distress than the middle-bedtime group.
- p-value A number from a statistical test that shows how unusual a result would be if there were really no effect. Values above .05 here mean the link was not clear enough to rule out chance.
Source: Lauren D. Asarnow, Eleanor L. McGlinchey, Allison G. Harvey. The Effects of Bedtime and Sleep Duration on Academic and Emotional Outcomes in a Nationally Representative Sample of Adolescents. Journal of Adolescent Health (2013). doi:10.1016/j.jadohealth.2013.09.004