Cognitive Function After Sleep Deprivation

Dr Andrew Scott of the Thoracic and Sleep Group Qld (TSGQ) has reviewed the current literature on sleep deprivation and cognitive ability compiling this comprehensive overview on the latest medical research.

It’s obvious to most, that sleep loss impairs cognitive ability including reaction time, attention, concentration and memory. Interestingly enough however cognitive decline is not predictable. In experimental studies, subjects put through sleep loss suffer period of dramatic cognitive decline that are interspersed with episodes of normal functioning, referred to as cognitive or state instability. Further to this, cognitive ability dramatically changes between subjects as well following sleep deprivation. In one such study for example, driving ability was shown to decline at significantly different rates with inter-subject analysis. Some subjects experience a marked decline in performance only one night after partial sleep deprivation (2 hours reduced sleep time) while others show slow and steady declines in performance as the partial sleep deprivation continues. There have been subjects also that show no significant loss in driving ability after days of partial sleep deprivation. Researchers at the University of Pennsylvania are now trying to identify genes that help protect against functional decline after sleep loss.

Intra-subject cognitive instability however is being explained by identifying brain networks that are employed during different tasks. Specifically, regions of the brain including the right middle frontal gyrus, the right and left inferior parietal lobes and basal ganglia structures are involved in simple reaction time tests. This frontoparietal sustained attention network is one that shows a higher degree of variability in those patients who are sleep deprived, resulting in a 150% increase in lapses in concentration. This network is thought to become transiently deactivated after sleep loss, as midline regions of the brain become activated.  

The midline “default mode network” of the brain is closely related to “passive” tasks as opposed to “active” or cognitive tasks. The transient switching between these two networks after sleep is likely due to the conflict between the drive for sleep (set by sleep deprivation), and the natural circadian urge for wakefulness, one team theorises.

A related study used a battery of cognitive tasks to assess performance on people who were sleep deprived. The task battery was specifically designed to dissociate executive (ie. Prefrontal cortex activation) functioning from non-executive functioning. Interestingly, the results showed either no effect or even improvements in executive functioning after sleep deprivation, however overall performance on most tasks was significantly reduced due to severely impaired non-executive processing.

Cognitive Function: Control and Experimental Groups

Executive functioning increased after sleep deprivation (based on number of words recited and number of cluster switches made). However overall performance decreased due to significant increases in lapses and decreases in memory association tasks. Both research groups concluded similarly, that sleep loss forces a shift in the way the brain processes information. In the case of prefrontal activation, this seemed to increase in order to compensate for a marked decrease in automatic cognitive processes; however other studies have found inconsistent results to this. Overall, sleep deprivation slows average Reaction Time by up to 30% and increases attention lapses by 150%. While fastest and slowest reaction times stay relatively stable between sleep deprived and well rested subjects, variability between reaction times increased in those who were sleep deprived.

Conclusions:

After sleep deprivation, brain networks seem to switch between the active network associated with cognitive tasks, and the ‘baseline’ state associated with passive tasks, and more specifically correlated to inattentiveness. Furthermore, the decrease in non-executive processing (including that of reaction time and attentiveness) outweighs the increase in executive processing after sleep loss and thus significantly reduces overall performance in executive processes.
•    Sleep deprivation does not so much cause a constant suppression of cognitive ability but more so an unpredictable capacity to perform tasks to a set standard.
•    Sleep loss effects people differently, with some only mildly affected while other are severely impaired
•    Sleep loss leads to unpredictable performance. Some of the time a person may function normally after sleep loss while the rest of the time they function significantly below par.
•    State instability is the term used to describe unpredictable performance and is the result of different brain Networks becoming alternatively activated, causing lapses in concentration and attention, leading to errors and slow reaction time.
•    Executive functioning (through the use of the prefrontal cortex) is not impaired as such, however lapses in concentration and attention lead to poor judgement and uninformed decisions.

References:

1.    Tucker, A, M., Whitney, P., Belenky, G. et al. Effects of Sleep Deprivation on Dissociated Components of Executive Functioning. SLEEP 2010; 33:47-57
2.    Drummond, S.P., Bischoff-Grethe, A., Dinges, D. et al. The Neural Basis of the Psychomotor Vigilance Task. SLEEP 2005; 28:1059-68