"Analyses of the oral fluid samples obtained from daytime drivers indicated an overall drug use prevalence of 11 percent, and for nighttime drivers, 14.4 percent (Table 19). This includes illegal, prescription, and over-the-counter drugs combined. This overall difference between day and night is statistically significant (p < .01).
"In examining the prevalence of drugs by class (Table 31), marijuana was identified in 3.9 percent of daytime drivers and 6.1 percent of nighttime drivers. Sedatives were found in 1.6 percent of daytime drivers and in 0.6 percent of nighttime drivers. Conversely, stimulants were found in 1.6 percent of daytime drivers but in 3.2 percent of nighttime drivers."
Lacey, John H.; Kelley-Baker, Tara; Furr-Holden, Debra; Voas, Robert B.; Romano, Eduardo; Ramirez, Anthony; Brainard, Katharine; Moore, Christine; Torres, Pedro; and Berning, Amy , "2007 National Roadside Survey of Alcohol and Drug Use by Drivers," Pacific Institute for Research and Evaluation (Calverton, MD: National Highway Traffic Safety Administration, December 2009), p. 111.