Three consecutive nights of restricted sleep: effects of morning caffeine consumption on mood, reaction time and simulated driving performance

Abstract

Introduction: Sleep is a fundamental biological requirement, essential for optimal health. Individuals unable to achieve adequate amounts of sleep may experience a range of negative behavioural and psychological consequences (e.g. increased daytime sleepiness, altered mood, reduced cognitive performance, driving impairment). Caffeinated products are often consumed as a popular countermeasure to reduce sleepiness, enhance mood, and improve cognitive functioning. However, the efficacy of caffeine to exert these effects after consecutive nights of restricted sleep is poorly understood, particularly in relation to performance on complex applied tasks (e.g. driving a motor vehicle). Therefore, the aim of this study was to investigate the effects of three consecutive nights of restricted sleep on subjective ratings of mood, cognitive function, and simulated driving performance and assess the ability of a morning dose of caffeine to attenuate any effects associated with sleep restriction. Materials and methods: Twenty healthy habitual caffeine consumers (11 females; age: 23.3±5.7 y; BMI: 22.3±3.5 kg·m-2; caffeine intake: 204±89 mg·day-1; Mean±SD) who had normal sleeping patterns (≥8 h sleep/night) participated in this double-blind, placebo-controlled, randomised study. Following one night of normal sleep (Day 0: ≥8 h time in bed (TIB)), participants underwent three consecutive nights of restricted sleep (Day 1, 2, 3: 5 h TIB). Participants received caffeine (200 mg; n=10) or placebo (n=10) capsules, together with a decaffeinated coffee and standardised breakfast each morning. All participants received caffeine (100 mg) capsules to consume in the afternoon of each trial day. On Day 0, 1 and 3, participants completed visual analog scales to measure subjective ratings of alertness, concentration and tiredness, before and 1 h after capsule administration. Cognitive function was examined 1 h after capsule administration using a computerised Choice Reaction Time (CRT) task. Response speed and accuracy were the outcome variables. Analysis of response speed was conducted using both traditional central tendency measures (comparing mean and variance) and ex-Gaussian distributional analysis. Driving performance was assessed using a 30 min simulated driving task. Lateral (standard deviation of lane position [SDLP] and total number of line crossings [LC]) and longitudinal (standard deviation of speed [SDSP]) measures of vehicular control were the outcome variables. Results: Alertness and concentration significantly decreased, and tiredness increased across the three days of sleep restriction (all p's< 0.001). Caffeine only marginally alleviated these effects. No differences were observed between treatments or across trial days for response speed and accuracy on the CRT task, irrespective of the analytical approach employed. Likewise, no significant differences were observed between groups or across trial days for lateral (SDLP, LC) and longitudinal (SDSP) measures of simulated driving performance. Conclusion: Overall, results from this study indicate that three consecutive days of sleep restriction may influence subjective ratings of alertness, concentration and tiredness, but does not appear to impact CRT or simulated driving performance. Caffeine may alleviate some of the negative subjective effects imposed by restricted sleep, but the efficacy of caffeine to attenuate performance changes in cognitive function and driving performance were unable to be observed in this study.