Impact of acute caffeine intake on local tolerance to cold before and after total sleep deprivation

Total sleep deprivation (TSD) alters local cold tolerance and could thus increase the risk of cold injury. We evaluated the impact of acute caffeine intake, the main countermeasure to TSD-related deleterious effects, on local cold tolerance before and after TSD. Thirty-six healthy subjects underwent two TSD protocols (i.e., continuous wakefulness), with randomized crossover intake of acute caffeine or placebo (2.5 mg/kg) administered twice during wakefulness. Before and after 33 h of TSD, finger (index and annular) temperature and skin blood flow were assessed during cold-water immersion (CWI, 5 degrees C, 20 min) followed by 20 min of rewarming in ambient air. We showed no significant effects of TSD on mean finger temperature during CWI in the placebo condition, but a significant reduction of the minimal temperature (8.86 degrees C +/- 0.35 degrees C vs. 8.64 degrees C +/- 0.27 degrees C, p = 0.02). During rewarming, we showed a reduction in temperature in the placebo condition (p = 0.02 for the mean temperature and p = 0.03 for the maximal) and an increase in the skin blood flow disparity between fingers at the four points of laser speckle rewarming measurements (p = 0.03). After TSD, acute caffeine intake (vs. placebo) increased mean (+2.11 degrees C +/- 0.21 degrees C, p = 0.01) and minimal (+0.61 degrees C +/- 0.10 degrees C, p = 0.02) finger temperatures during CWI, and improved rewarming after CWI (mean and maximal temperatures) (+2.28 degrees C +/- 0.08 degrees C, p = 0.01, and +2.06 degrees C +/- 0.12 degrees C, p = 0.02, respectively). Before TSD, acute caffeine intake significantly increased (vs. placebo) mean temperatures during CWI (p = 0.03) and reduced pain from the onset (p = 0.03) to the end of CWI (p = 0.02) and the first 2 min of rewarming (p = 0.04). There was also a significant main effect of habitual daily caffeine consumption on minimal finger temperatures during CWI, which decreased significantly between 0 and 600 mg consumption (R2 = -0.43, p = 0.01), independently of the effects of day (before and after TSD) and treatment (caffeine and placebo conditions). These findings suggest that acute caffeine intake could be a protective countermeasure to local cold tolerance, particularly during TSD. However, habitual daily caffeine consumption is a factor of individual variability that should be recorded during CWI protocols. Clinical trial NCT03859882.