Wide awake at bedtime? The effects of caffeine on sleep and circadian timing in teenagers - a randomized crossover trial

Background: Adolescents frequently consume caffeine with unknown consequences on sleep and circadian rhythms. In adults, the evidence indicates that caffeine acutely reduces homeostatic sleep pressure and delays the circadian timing system. Objective: Here, we investigated the acute effects of caffeine intake on the developing sleep-wake regulatory system of teenagers. Design: In a double-blind randomized crossover laboratory study, 18 teenagers (16.1 ± 1 years old, pubertal development scale [PDS]: 2.76 ± 0.35) ingested 80 mg caffeine (vs placebo) four hours prior to bedtime. Until bedtime, participants regularly filled in the Karolinska Sleepiness Scale and gave saliva samples to measure melatonin secretion. During nighttime, we quantified homeostatic sleep need by the electroencephalographically derived amount of slow wave sleep duration (SWS). After sleep, participants rated sleep quality by the Leeds Sleep Evaluation Questionnaire. Results: While participants felt less sleepy after caffeine vs. placebo (P=0.038), their ratings of sleep quality were not strongly affected by the treatment. However, objectively, SWS was on average reduced by ~20 min after caffeine vs. placebo (P=0.026). This caffeine-induced reduction was more pronounced in those individuals with more SWS under placebo (regression: P=0.042; standardized beta=0.622, P=0.011), even if controlling for habitual caffeine intake or pubertal stage (PDS). In melatonin onsets we observed both delays and advances in response to caffeine. This variance could partly be explained by differences in relative dose (i.e., mg caffeine/kg of bodyweight): the higher the relative dose, the more likely were delays (regression: P=0.01; standardized beta=0.592, P=0.01). Conclusions: In teenagers, evening caffeine intake of already 80 mg (i.e. ~8fl oz of common energy drinks) is sufficient to promote alertness at the costs of subsequent sleep. These costs might be more pronounced in adolescents with a higher need for SWS. Moreover, caffeine might disturb the circadian timing system, consequently hampering the balanced interplay of sleep-wake regulatory components.