summarizes the main findings. The evidence suggests that poor sleep leads to drug use in adolescents, and that both sleep and drug use spread through social networks. We show (for the first time, to our knowledge) a mediation effect in which the spread of one health behavior affects the spread of another. Specifically, a person's risk of drug use increases if his friends sleep poorly, and this effect is mediated in part by the spread of poor sleep behavior from one person to another.
It is well known that both sleep and drug use can be influenced by social pressure 
. But behaviors do not exist in isolation: one behavior can influence another both within and between individuals. Hence, beyond social influence, the present study is the first to consider the coevolution of two spreading processes on a fully observed network. By tracking subjects over time, our data show not only the pattern of growth of two independent health behaviors, but go further to reveal a mechanism by which one may encourage another. One possible explanation for our results may be the effect of poor sleep on decision-making and behavior regulation. Poor sleep in adolescents has been shown to decrease behavior regulation, including impulse control, emotion regulation, and behavioral flexibility 
, as well as health-risk behaviours, such as suicidality and substance use 
, and poor school performance 
The present study was limited to survey questions gathered about marijuana use. It is common, however, for marijuana to act as a ‘gateway’ increasing risk for youth to progress to other, more serious drugs 
. The National Center on Addiction and Substance Abuse at Columbia University recently reported that nearly half of full-time students binge drink and/or abuse prescription and illegal drugs 
. From 1993 to 2005, abuse of prescription opioids, or pain killers, increased 343%; abuse of prescription tranquilizers such as Xanax and Valium rose 450%; and abuse of prescription stimulants such as Adderall was up 93.3%. The present study was unable to account for the effect of other substance use on an individual's sleep patterns, or the progression of friends' substance use on the network. Examination of the spread of other substances, such as amphetamines that are known to disrupt sleep, would be a natural extension of the present findings.
We also show a number of novel network effects. In contrast to prior studies that showed associations in behaviors and affective states extending up to three degrees of separation 
, here we show they extend up to four degrees for both sleep and drug use. We also show that people with high network centrality are at greater risk of poor sleep, which in turn affects drug use. This is consistent with the finding that negative outcomes for both sleep and drug use spread more reliably than positive ones; more centrally located individuals are in a structural location that makes them more susceptible to behavioral contagions, since they sit at a sort of “crossroads” in the network.
These results pave the way for further analysis of other negative health outcomes that may be mediated by sleep, such as alcohol use. It is likely that the negative effects of poor sleep that lead an adolescent to try marijuana would influence decisions to try alcohol in similar ways. We also hypothesize that behaviors that have already been shown to spread through social networks, such as obesity 
, drinking 
, and smoking 
, may be good candidates for such a mediation analysis. And, apart from sleep, these data suggest a way forward in identifying the causal processes that underlie important behavioral contagions. For example, we could use the same methodology to study whether the spread of eating certain types of food (like junk food) mediates the spread of obesity 
Our findings suggest a number of directions that might help policy-makers and researchers to improve anti-drug campaigns. First, efforts to increase sleep in adolescents may help to reduce substance use in this population. Although the present findings show that poor sleep behavior has a stronger influence on the network compared to good sleep behavior, lessening the influence of poor sleep behavior may be an effective mechanism for decreasing substance use. One possible intervention would be a napping program during or after school. Napping has been shown to improve memory 
, alertness 
, and creativity in young adults 
. Helping adolescents to get more nocturnal sleep may be challenging. Although the present analysis did not examine morning rise time (the information was not available), future studies should investigate whether the majority of adolescents who are vulnerable to the negative consequences of sleep loss for drug use also have the greatest level of delayed sleep phase. Perhaps, for these individuals, a systematic daytime sleep regimen would be advisable, and the results reported here suggest that benefits of such an intervention could spread to influence friends.
Our results also show that adolescents who are most centrally located in the social network are not only more influential but also the most vulnerable to poor health outcomes. This suggests that anti-drug interventions targeting central individuals may be more effective for reducing drug use across the whole network. In the same vein, sleep interventions may benefit from targeting individuals who are more central to the network, and such interventions could spill over and help reduce drug use in many other individuals. Studies of vaccinations have shown a linear effect between success of vaccination and number of connections 
. Furthermore, costs of interventions can be reduced substantially when more central individuals are treated 
. Targeted sleep interventions might be similarly effective, helping to immunize a whole population against drug use. People are connected, and so their health behaviors are connected.