Acute sleep deprivation is a common occurrence in the general population  including elite athletes. Such deprivation has been shown to affect some, but not all, physical and skill executions [15, 20–22]. However, quantifying an effect in a team sport can be difficult. The repeated passing skill test we described herein is simple to perform, has sport-specific relevance and appears to be highly reliable across repeat testing. It is not however a one off, high-level performance task, rather a repeat of 20 fairly simple tasks, alternating passing sides. While we don't claim it to be in any way, yet, a valid performance measure it did reveal some interesting differences across acute sleep deprivation and across caffeine and creatine treatments.
In line with observations in other skill and psychomotor testing acute sleep deprivation reduced the accuracy over repeated trials. There was a general trend to a drop-off in accuracy latter in the repeats (second 10 of the 20 repeats). Whether this is a greater susceptibility to mental fatigue or not remains an interesting question, as does whether single skill repeats separated by more recovery time or by a similar physical activity with no real skill requirement would show a deficit in performance or not. In non-sport related psychomotor trials there is little evidence that a single episode of sleep deprivation produces significant deficit in a single task ; however across repeat tasks it is perceived that much greater effort is needed to maintain concentration .
Acute sleep deprivation has little effect on weightlifting performance , but can influence mood negatively  which may be a driving feature in mental performance changes. Caffeine, for example, has been shown to improve both mood and mental function following sleep deprivation . It is not known how much mood and other cognitive function, particularly motivation on repeat skill tasks, interact. At the doses and administration time of caffeine use in this study we saw no evidence of an effect in non-sleep deprived subjects; however, there was a clear amelioration of skill performance deficit from the sleep-deprived trials with placebo administration. The psychostimulant effects of caffeine appear to be related to the pre and post synaptic brakes that adenosine imposes on dopaminergic neurotransmission by acting on different adenosine receptor heteromers , although numerous mechanisms are likely to be involved.
We did not see a dose related effect with caffeine supplementation, with 1 mg/kg and 5 mg/kg producing similar effects, nor did we see high individual variance (i.e. responders and non-responders). The absorption of caffeine in plasma following consumption has been estimated at between 30 and 90 min with half life of several hours , so the time between consumption and testing (90 min) in this study may have been too long to see all effects of differing caffeine dose, or any effect on non-sleep deprived performance. Nonetheless, at 90 min there was still clear evidence of a reduction in the effect of sleep deprivation on the skill measured and no evidence this was different between the 1 and 5 mg/kg dose.
Subjectively, a number of the subjects reported feeling slightly nauseous and anxious following the 5, but not 1, mg/kg administration of caffeine suggesting in other ways there were dose differences. Effective doses of caffeine (and their dose response nature) remain contentious in literature [1, 5, 6, 27] possibly reflecting larger inter-subject variability in responses and different sensitivities of various physical and behavioural expressions. The subjects in this study were not regular caffeine users so arguably may have been more sensitive to lower doses than would be seen in more regular consumers. Certainly in the study herein 1 mg/kg was as effective as 5 mg/kg and from a practical perspective runs less risk of undesirable dose related side effects.
Chronic creatine supplementation has been shown to address certain aspects of sleep deprivation linked and other pathophysiology linked cognitive deficits [8, 9, 11, 13, 14, 19], although very low dose chronic supplementation does not appear to improve function in non-sleep deprived healthy subjects . Sleep deprivation is associated with a reduction in brain stores of phosphocreatine  and certainly in some disease states depletion of high energy phosphate stores has been measured, associated with cognitive deficit, and alleviated to some extent by creatine supplementation [13, 14, 29]. Interestingly, if there is an energy deficit associated with sleep deprivation then it seems logical to contend that repeat trials would be more susceptible than one off tasks. Our results and indeed other work on sleep deprivation do fit this pattern. If such depletion occurs and is acute, it also stands to reason that acute supplementation (as opposed to longer protocols) would address any associated deficit (given that brain uptake is not a time limiting factor). Little, if any, attention has been given to acute dosing with creatine, mainly because it is assumed that its effects come from a gradual build up of stores over time. We demonstrate here that an acute dose of creatine can ameliorate sleep deprived deficits in repeat skill performance trials. Again this possibly reflects the repeat nature of the trials and may not be observable in an acute one off mental skill performance.
Further in contrast to caffeine administration, the creatine dose of 100 mg/kg appeared to elicit a trend towards greater effect in skill performance than 50 mg/kg dosing, thereby suggesting potentially a dose dependent response. As in the case of caffeine we observed no individual variability suggestive of responders and non-responders or differential dose susceptibility, and no adverse effects were reported to us by the subjects. Clearly at the level of muscle function there does appear to be a division into responders and non-responders to longer term supplementation with different creatine protocols . It is possible that this would be similar with longer term supplementation aimed at skill improvement, or alternatively brain-related creatine stores may operate slightly differently to muscle.
Acute sleep deprivation has been demonstrated in some studies to have small disruptive effects on basal hormonal concentrations [30, 31]. Although salivary cortisol appeared to be elevated with sleep deprivation, this result did not reach statistical significance. Interestingly the higher dose of caffeine was associated with significant elevation in pre-trial cortisol, but not testosterone. High doses of caffeine have previously been demonstrated to acutely increase cortisol and, to a lesser extent, testosterone [20, 32]. Whether such elevations have any significance in outcome is unknown. Cortisol is associated with arousal but also with anxiety . Unfortunately we did not concurrently measure salivary alpha amylase in this study, which may also be a useful marker with respect to system arousal . Testosterone was unaffected by sleep deprivation and by all treatments except the high dose of creatine, where there was a trend towards higher concentrations. We do not have useful speculation as to why this increase was seen, although it was across all subjects. Still, the increase was relatively small in magnitude and we doubt at this stage that it has any real physical or behavioural consequence. As we used saliva measures we cannot rule out some local oral cavity artefact effect of creatine. Free testosterone levels have, however, been linked to intra-individual variance in short timeframe muscular power , and long-term creatine supplementation has been reported as influencing testosterone metabolite pathways , so the observation is perhaps worthy of some follow-up.
Little has been published on acute creatine use as it has primarily been regarded as a longer term supplement to muscular function gain. In terms of brain and behavioural function it would appear it have some acute effects of value. It is also possible that the observed effects of caffeine and creatine reported in this and other studies are potentially summative and thus, would seem a logical progression for research.