Sleep loss and 'executive thinking'
Apart from causing 'sleepiness', sleep loss particularly affects the more subtle forms of human behaviour known as higher 'executive' function, largely controlled by the prefrontal cortex - a brain region at its most advanced in humans. It is the hardest working part of the cortex during wakefulness - which may be why it seems so vulnerable to sleep loss. Here, sleep loss effects include: rigid thinking, reduced verbal fluency, perseveration, impaired working memory, inability to deal with novelty and the unexpected, and less inhibited social behaviour. Interestingly, unlike 'sleepiness' these deficits show much less of a 24 hour circadian rhythm, but steadily worsen with sleep deprivation. Contemporary '24/7' society requires many people to work with sleep loss, whether this be through shift-work, long working hours or simply from late night socialising. Despite the large number of sleep loss studies that have been undertaken, these aspects of sleepiness are not so well understood, which is the reason why we have been investigating this topic over the last 10 years or so.
'Distractibility' under Monotonous Work Settings
Another aspect frontal lobe function is the ability to maintain attention to a task when there is competing distraction. Sleepy people seek stimulation to remain awake, and will look around for something new to look at, becoming more easily distracted from a task, especially if it is dull and boring. Most laboratory studies measure sleepiness using these tedious but sensitive tests, within a sterile room, purposely devoid of any distractions. However, by creating such an environment, few, if any studies of sleep loss have assessed the impact of distractions in any systematic way. We find that sleepiness increases vulnerability to distraction and that the propensity to distraction is, itself, an important measure of sleepiness.
The Social Neuroscience of Sleep Loss
A new area of research into this executive function is 'social cognition' - the ability to comprehend non-verbal, social signals from others and make appropriate judgments. The effects of sleep loss on these abilities is a new research programme, well under way.