Figure 1. Caffeine concentrations in saliva on 4 test days. Entries show means, and error bars represent standard errors. Samples from 7:30 AM to 2:00 PM were collected in the laboratory, and the samples at 6:00 and 7:00 PM were collected at home. PC = placebo maintenance followed by 3 x 250-mg caffeine challenges on the test day. C300 = 300 mg/day of caffeine at home followed by caffeine challenge on the test day. C600 = 600 mg/day at home followed by caffeine challenge on the test day. PP = placebo at home and placebo on test day. Base 1, Base 2, Base 3 = saliva samples taken immediately before taking a caffeine or placebo capsule. PostC = samples taken 1 hour postdrug. Stress and Recov = samples taken at the end of a 30-minute behavioral stress period or after 30 minutes of recovery.