Effects of Caffeine Intake on Self-Administered Sleeping Quality and Wearable Monitoring of Sleep in a Cohort of Young Healthy Adults

Background/Objective: Chronic sleep deprivation can lead to adverse health effects and therefore represents a public health burden While caffeine is a widely used stimulant, the relationship between caffeine consumption and sleep remains uncertain. Moreover, many studies might be subject to methodological bias, as invasive sleep measurements may themselves serve as confounders. The aim of the SleepSmart study was to assess the impact of caffeine consumption through coffee and energy drinks, utilizing both smartwatch data and questionnaire responses. Methods: The SleepSmart study is an observational cohort study conducted at LMU Hospital Munich, Germany, from July 2024 to January 2025. The study investigated two intervention groups: (1) coffee and (2) energy drink consumption. Each intervention lasted 1 week, with days 1 to 3 serving as a reference period (water consumption to adjust for increased fluid intake) and days 4 to 6 serving as the intervention period (consumption of an equivalent of 240 mg of caffeine per day, 3 h before bedtime). Data was collected through self-administered questionnaires and wearable devices. The primary endpoint was sleep duration. Objective measures of sleep (duration of light/deep sleep, duration of wake phases, heart rate) and self-assed quality of sleep (Pittsburgh Sleep Quality Index) served as secondary endpoints. Results: For the primary endpoint, we did not find a significant difference in average sleep duration (baseline [hours] 6.7, coffee 6.9, energy drink 6.7, p-value 0.183). Similarly, secondary endpoints related to sleep measures showed no significant changes in the duration of light/deep sleep (coffee [min]: 242.2, reference coffee [ref_C] 255.7 I energy drink: 237.5, reference energy drink [ref_E] 261.3), time awake (coffee 22.7, ref_C 23.4 I energy drink 21.3, ref_E 18.9), time to fall asleep/wake up (coffee 4.2, ref_C 4.0 I energy drink 4.4, ref_E 2.7), or average heart rate (coffee [bpm] 62.2, ref_C 62.1 I energy drink 62.6, ref_E 62.3)—neither between the two intervention groups nor compared to the reference period. However, self-assessed sleep quality revealed a decrease in perceived sleep quality, with reports of poor sleep increasing by 8% after coffee consumption (rather bad/very bad sleeping quality: 29.7%) and 20% after energy drink consumption (46.6%), compared to baseline data (21.6%). Conclusions: The SleepSmart study introduces wearable sleep tracking as an innovative, low-interference method for objectively recording sleep data. While wearable data did not indicate significant sleep deterioration in the group of young, healthy adults, caffeine appeared to negatively impact the subjective perception of sleep in the study cohort.