By Maria M. Hadjimarkou, PhD - Lecturer School of Psychology University of Sussex
Several vital functions take place when we sleep, such as the optimisation of our immune system, the clearing of toxic brain waste and the encoding of new learning. Several studies find that not sleeping enough compromises several of these functions. Many of us reduce our time in bed for work or entertainment and often sleep less than 6 hours each night. Of course, a short sleep may be enough for some, but for the majority, it is insufficient, and this is evident in several ways. One of these is our performance in cognitive tasks that require our attention and engagement for a period of time.
Attention can be thought of as a prerequisite to learning, so we expect someone to pay attention in order to understand and learn. Sustained attention, also known as vigilant attention, implies maintaining attention for a period of time while doing a task. Perhaps it is easier to understand this if you think of driving, which requires attention from the moment we sit in the driver’s seat until we take the keys out of the ignition. Although driving ultimately becomes automatic and many times we may loosen our focus, it does require sustained attention. Similarly, sustained attention is required for work when we do a task or operate some sort of equipment.
It is estimated that several errors or accidents in the workplace or on the road are due to sleepiness or compromised attention due to reduced sleep. In the United States, every ten years the Centre for Disease Control and Prevention (CDC), publishes a set of goals that reflect their strategy towards a healthier and happier nation. This initiative termed ‘Healthy People’ (https://health.gov/healthypeople) now includes sleep as a new area, which among others includes a specific target to reduce the rate of motor vehicle accidents due to drowsy driving.
Several experiments trying to assess the impact of sleep restriction (fewer than 6h of sleep) or sleep deprivation (no sleep) on vigilant attention have been using driving simulators in the lab or cognitive tasks involving shapes, numbers etc. More often though they use a computer screen and the presentation of a stimulus to which participants must respond by pressing a key on the keyboard as fast as they can. The most prevalent of these tests is the Psychomotor Vigilance Test (PVT). The PVT involves paying attention and responding to a stimulus that appears at random times by pressing a key on the keyboard or a keypad. The whole task lasts for 10 minutes. How can they tell anything about our attention with this setup? Well, they measure the amount of time it takes to respond (notice the stimulus on the screen and press a key), known as reaction time. They may also count the errors that we make when we hit the key before the stimulus appears or fail to hit the key on time. You may think that this has nothing to do with a real-life situation, however, it is considered a reliable way to estimate our vigilance with a high degree of applicability.
Performance in this and other similar tasks deteriorates in both those sleep-deprived and sleep-restricted, compared to those that get sufficient sleep (around 8 hours). This difference in performance is evidenced by increases in reaction time and by increases in the number of mistakes or lapses (see graphs). In addition, although everyone is expected to show a drop in performance as the task progresses due to fatigue, what is known as time-on-task, those with less sleep get much worse compared to those with sufficient sleep. It is also interesting to note that performance in such tasks varies, depending on the time of day. Remember that we are diurnal, so we are expected to be alert and vigilant during the day whereas at night we are supposed to be sleeping and dreaming away! The impact of circadian rhythm is evident in these experiments, as the greatest drop in performance is seen when people are tested at night and especially in the early post-midnight hours. Thus, working or driving during that time while sleep-deprived, worsens our ability much more than if we were doing these tasks while sleep-deprived, but during the day. One interesting twist to this is the difference between what people report and how they perform in these tasks. Perhaps you have experienced this if you were out with friends who had a few drinks, and you offer to drive but they respond confidently that they feel fine and they are very capable of driving. A similar theme emerges in studies on sleep deprivation and cognitive functioning, where sleep-restricted people say that they feel fine, but when they do the task it is evident that their ability to pay attention and respond appropriately is very much compromised. Importantly, these decrements in performance correspond to the time of sleep deprivation, in a dose-response way, so that the more time they spend awake, the worse their performance will be. In some cases, limiting sleep to 4 or 6 hours per night for a few consecutive days, resulted in deficits in performance similar to those seen in people who were sleep-deprived for 1 or 2 days.
Of course, as with all other things we have been discussing, sleep deprivation may not have exactly the same effect on each and every one of us. Indeed, some may be more susceptible than others, and some researchers are evaluating the differences in our genes that may be responsible for this difference in susceptibility. Perhaps identifying those more vulnerable will be helpful in some way, however, this effort is still at the exploratory level. For now, it may be wise to allow sufficient time for sleep, to ensure that we are getting enough on a regular basis. Who knows, we may see improvements in performance in various aspects of our lives ☺
The mean number of lapses or errors (not responding in time) in those with regular sleep (the control group in blue) and those sleep-deprived (in red), with a night of recovery sleep in the end, showing a decent recovery in performance. The long grey bars indicate night-time sleep whereas the short grey bars indicate night-time without sleep for those in the sleep-deprived group (Hudson, Van Dongen and Honn, 2020).
Errors in the PVT as a function of time in bed (TIB) after 8 consecutive days of tracking performance, with those who spent less TIB performing worse than those with more TIB (Hudson, Van Dongen and Honn, 2020).
Key Reference: Hudson AN, Van Dongen HPA, Honn KA. Sleep deprivation, vigilant attention, and brain function: a review. Neuropsychopharmacology. 2020 Jan;45(1):21-30. doi: 10.1038/s41386-019-0432-6.
Posted in sleep on Mar 01, 2023