The hypothesis underlying this study was that acute SD for 24 h negatively influences daytime function and increases hormone levels in healthy volunteers. As expected, the fatigue level increased and alertness decreased over time in our subjects. After a total SD of 24 h, subjects recorded a VAS score of 95 (extremely fatigued), and an SSS alertness score of 6 (foggy, losing interest in remaining awake, and slowed-down state). The CPT used in this study measures a person's sustained and selective attention and working memory. Our results showed that at easier steps (levels 1 and 2), the mean scores of correct responses were not significantly decreased after SD, but they were significantly reduced (decreased by 2.2%) at the most difficult step (level 3). The mean scores of error responses were significantly increased at all levels (increased by 69.5-136%) after SD. These findings show that attention and working memory are impaired after acute SD of 24 h, with the performance decrements being more dramatic for more complex tasks. Interestingly, subjects exhibited a relatively good performance in correct responses at easier steps after SD, but they made errors frequently even at easier steps as well as most difficult level. The findings suggest that the SD condition results in the subject being more susceptible to making an error.
Regardless of the task, cognitive performance progressively deteriorated for increasing time spent on the task; this is a classic "fatigue" effect that is exacerbated by sleep loss.9
Previous findings that performance on even brief cognitive tasks that measure attention and working memory was sensitive to SD2,10,11
support our results.
This study showed that acute (24 h) SD induces increases in the serum concentrations of stress hormones, and suggests that SD itself is the stress-eliciting situation. Although the increased concentrations of hormones after SD were still within normal ranges, statistically heightened levels of hormones after SD may indicate the important effect of sleep on stress hormones. Some studies have shown that stress hormones may interfere with memory and other cognitive functions. A significant relationship between pre-retrieval stress or high cortisol levels and impaired memory retrieval has been reported consistently.12,13
Acute elevations of exogenous glucocorticosteroids were shown to impair working memory without affecting declarative memory.13,14
A significant association was found between insomnia duration and left dorsomedial prefrontal damage in patients with focal brain lesions.15
The gray-matter volume in the left orbitofrontal cortex is reduced in chronic insomnia, and was found to be strongly correlated with the subjective severity of insomnia.16
Although the aim of the present experiment was to examine 24 h SD, significant deficits in attention and working memory after SD suggested that acute SD causes a deterioration in prefrontal function.
While cross-sectional and longitudinal studies point to a U-shaped relationship between self-reported sleep duration and the risk of type 2 diabetes mellitus,17,18
it was recently reported that prebreakfast concentrations of blood glucose and other inflammatory markers were not changed after restricting sleep (to <4.5 h/day) for 2 nights.19
We also found unchanged serum concentrations of glucose after SD. Whether or not acute SD alters glucose metabolism therefore remains uncertain. Additional measurements of other hormones such as insulin and glucagon would be helpful to clarify this issue.
Insufficient sleep and poor sleep quality are risk factors for inflammation-related conditions.19-21
Total SD for 40 h caused significant increases in E-selectin, intercellular adhesion molecule 1, interleukin-1b, and interleukin-1Ra, and decreases in CRP and interleukin-6, suggesting that sleep loss triggers a stress response that includes stimulation of both pro- and anti-inflammatory proteins.22
In contrast, we did not find any differences in the serum concentrations of ESR and CRP, which are nonspecific measures of inflammation. It is not clear whether the difference in these results is attributable to the duration of sleep deprivation or the specificity of inflammatory markers.
While this study has not revealed a direct relationship between stress hormone levels and cognitive function, we did find that acute SD for 24 h significantly heightened the levels of stress hormones and caused a deterioration of cognitive function. The acute SD condition appears to render the subject more susceptible to making an error.