Effects of caffeine ingestion on the diurnal variation of cognitive and repeated high-intensity performances

The purpose of this study was to evaluate the effects of caffeine ingestion on the daily variation of cognitive (i.e., reaction time (RT), attention) and repeated high-intensity exercise performances. Fifteen active males (age: 20 +/- 1 years, height: 174.3 +/- 4.3 cm, body-mass: 70.8 +/- 15 kg) performed cognitive and physical tasks under two different circumstances [after a placebo or caffeine ingestion (6 mg/kg of body-mass)] at six different time-of-day (07 h00, 09 h00, 11 h00, 13 h00, 15 h00 and 17 h00) in a randomized double-blind balanced crossover design. During each session, RT, attention and 5-m multiple shuttles run test' performances were recorded. During both the placebo and the caffeine conditions, a significant diurnal variation was found with improvement of cognitive performances recorded at 11 h00 (e.g., RT: 0.37 +/- 0.02-s and 0.36 +/- 0.02-s for placebo and caffeine respectively) and 17 h00 (e.g., RT: 0.37 +/- 0.02-s and 0.35 +/- 0.03-s for placebo and caffeine respectively) compared to (i.e., worst performances) 07 h00 (e.g., RT: 0.41 +/- 0.02-s and 0.38 +/- 0.02-s for placebo and caffeine respectively) and 13 h00 (e.g., RT: 0.41 +/- 0.02-s and 0.38 +/- 0.02-s for placebo and caffeine respectively) (p < 0.05). For physical performance, improved values were recorded at 17 h00 (e.g., total distance: 730.00 +/- 43.92-m and 733.93 +/- 43.08-m for placebo and caffeine respectively) compared to 07 h00 (e.g., total distance: 698.14 +/- 45.39-m and 709.21 +/- 43.78-m for placebo and caffeine respectively) (p < 0.05). Compared to placebo, cognitive (e.g., RT: by 6.4% at 07 h00, 4.1% at 09 h00, 3.4% at 11 h00, 6.0% at 13 h00, 3.8% at 15 h00 and 3.8% at 17 h00) and physical (e.g., total distance: 1.6% at 07 h00, 0.9% at 09 h00, 0.1% at 11 h00 (p > 0.05), 0.5% at 13 h00, 1.0% at 15 h00 and 0.5% at 17 h00) performances increased at all time-of-day (p < 0.05). In conclusion, cognitive and physical performances are time-of-day dependent and caffeine is an effective ergogenic aid to improves both cognitive and physical performances especially at the moment of their lowest values.