Introduction: Caffeine is widely used to counteract performance impairments that occur following sleep restriction or deprivation. However, the efficacy of caffeine to attenuate performance decrements may depend on factors such as caffeine dose, timing of administration, habitual caffeine use, type and complexity of performance task and the amount of sleep lost. The aim of this study was to systematically review and meta-analyse published literature examining the effects of acute caffeine consumption on cognitive, physical and occupational performance in sleep deprived/restricted individuals.

Materials and methods: Electronic databases (PubMed (MEDLINE), Web of Science (via Thomas Reuters) and Scopus were searched to identify studies that measured cognitive, physical or occupational performance following either sleep restriction (≤6 h sleep within 24 h) or deprivation (≥24 h wakefulness) under control (placebo) and intervention (caffeine) conditions. Studies were eligible for inclusion if performance was assessed within 6 h of caffeine consumption and excluded if the effect of caffeine could not be isolated or if performance data were not adequately reported. Individual effect estimates (EEs) were calculated as Hedges' g for independent groups; if a study repeated the same performance test within a 6 h period (and no additional caffeine was administered between tests) the resulting Hedges' g values were combined into a single EE. Random effects meta-analyses were performed to determine intervention efficacy. Statistical significance was attained if the 95% CI did not include zero. Heterogeneity was assessed using the I2 index.

Results: 4351 records (excluding duplicates) were screened and 59 publications were reviewed. These publications provided 228 EEs on cognitive performance outcomes. Mean caffeine intake was 341±174 mg (range: 80-600) and total time awake was 32±13 h (range: 18-86). The cognitive domains assessed were attention, executive function, information processing, memory and reaction time; 'speed' and 'accuracy' data were handled separately within each domain. Simulated driving performance, an applied cognitive skill, was also examined. Caffeine decreased (i.e. improved) response time (43 EEs; g=0.71; 95% CIs 0.56,0.85; I2=48%) and number of lapses (22 EEs; g=0.73; 95% CIs 0.50,0.95; I2=60%) on attention tests, decreased response time (18 EEs; g=0.69; 95% CIs 0.20,1.18; I2=88%) and increased accuracy (17 EEs; g=0.47; 95% CIs 0.14,0.81; I2=78%) on executive function tests, decreased response speed on reaction time tests (12 EEs; g=1.12; 95% CIs 0.75,1.48; I2=74%), decreased response time (10 EEs; g=2.12; 95% CIs 1.11,3.12; I2=95%) and increased accuracy (32 EEs; g=0.49; 95% CIs 0.39,0.60; I2=22%) on information processing tests, and enhanced lateral (23 EEs; g=1.62; 95% CIs 1.28,1.96; I2=70%) and longitudinal (12 EEs; g=1.62; 95% CIs 1.14,2.10; I2=66%) vehicular control on simulated driving tests. No other performance outcomes were appropriate for meta-analysis. However, studies typically indicated a benefit of caffeine on memory (5 publications, 25 EEs), physical (13 publications, 41 EEs) and occupational (8 publications, 35 EEs) performance, although, the magnitude of effect and performance tests employed were heterogenous.

Conclusion: Results indicate that in situations where individuals experience inadequate sleep, the ingestion of caffeine is an effective strategy to enhance cognitive and physical function.