Figure 1
From: The impact of daily caffeine intake on nighttime sleep in young adult men

Average caffeine levels collected prior to and after nighttime sleep (grey bar) in the placebo (black open circles), caffeine (blue filled circles), and withdrawal (red semi-filled circles) condition (mean values ± standard errors). The x-axis indicates the mean time of day of sample collection and color-coded asterisks represent significant (p < 0.05) post-hoc comparisons of the interaction effect condition × time.