Caffeine is one of the most widely consumed psychoactive substances globally, used by millions to stay awake, boost productivity, and enhance performance. However, it also has a well-documented impact on sleep. Recent systematic reviews and meta-analyses provide a clearer understanding of both the beneficial and disruptive roles caffeine plays in everyday health and functioning.

A comprehensive review of epidemiological studies and clinical trials found that caffeine consumption generally delays sleep onset, reduces total sleep time and efficiency, and increases light sleep and nighttime awakenings (Clark & Landolt, 2017). The review also noted that caffeine decreases restorative slow-wave sleep and increases alertness-related brain activity during rest.

Notably, older adults appear more sensitive to these effects, and genetic differences also affect how strongly caffeine impacts sleep. Most studies were conducted in Western male adults, limiting broader applicability.

At the same time, caffeine shows potent advantages in situations involving sleep loss. A meta-analysis of 45 studies showed that caffeine significantly improves response times, accuracy, memory, and driving performance following sleep deprivation (Irwin et al., 2020). Caffeine improved attention (g = 0.86), executive function (g = 0.35), and vehicular control (g = 1.67 lateral, g = 1.60 longitudinal), supporting its role as a reliable countermeasure to cognitive deficits caused by lack of sleep.

Similar findings were echoed in another broad review showing that low to moderate doses of caffeine enhance alertness, reaction time, and physical endurance, particularly in occupational settings such as transport, healthcare, and emergency services (McLellan et al., 2016).

These dual findings reinforce caffeine’s paradoxical nature. While it helps maintain performance under sleep restriction, it can also degrade the very sleep that performance relies on. Effective use depends on timing, dosage, and individual sensitivity.