A study of over 12,000 students by McKnight-Eily et al., 2011  found that cigarette, alcohol and marijuana use declined by 8% to 14% when teens slept eight or more hours each night, with depression and sexual activity also declining by 9% to 11%.
Wahlstrom, et al., 2014 states “Previous national studies that assess the link between unhealthy behavior choices (e.g., drugs, alcohol, sexual activity) and depressive feelings and the amount of sleep time that students get have shown that 8 hours of sleep per night appears to be a critical point in whether or not high school students engage in such behaviors. In the table below, it is clear that as school starts later, an increasing number of high school students get 8 or more hours of sleep on school nights. For schools that started at around 7:30 AM, only a third to less than half of the student population obtained 8 hours of sleep or more.”
Wahlstrom, et al. adds, “one of the biggest adjustments affecting late adolescence is the significant change in chronotype with the delay of the intrinsic sleep phase. Coping with delayed sleep phase becomes problematic for teens who need to wake up early for morning classes, resulting in average weeknight sleep durations around 7 hours per night –much less than the 8.50–9.25 hours needed at this phase of their lives."
In another study of over 8,000 students (Mednick SC et al., 2010) the authors conclude: “The results indicate that interventions should focus on healthy sleep to prevent drug use.”  This study looks at how improving one student’s length of sleep can positively impact other students up to four degrees of separation.  It is a complex study looking at multiple variables, and yet the final recommendation was to target sleep to prevent drug misuse/abuse
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This review of Sleep and Risky Behaviors found that “a teen who is sleep deprived is less likely to completely perceive the risk associated with making a certain decision.  Risk-taking includes those behaviors that carry an increase in the possibility of death or illness.  These behaviors include engagement in tobacco, alcohol, or drug use; driving while sleepy; violence or sexual activities.  These behaviors have been associated with student sleep-wake patterns that result in less total sleep time.”  As you can see from the chart above, later start times result in more sleep for more students.
The CDC Youth Risk and Behavior Survey looked at the following risky behaviors (among others):  riding with a driver who had been drinking, drinking and driving, and texting while driving.  The study, conducted over 4 years and involving over 50,000 students, found that the "likelihood of each of the ...risk behaviors was significantly higher for students who reported sleeping ≤7 hours on an average school night."
The evidence is clear: starting school later reduces risky behaviors including drug and alcohol use.  It is a scientifically proven way to reduce risk at a reasonably low cost and therefore is a best practice.
Pam Hartnett MPH-RD