The effects of sleep deprivation on cognitive performance

We’ve all worked deep into the night to meet a deadline or had a spell of disrupted sleep during our working lives. But grogginess is one thing. Chronic sleep deprivation, which negatively impacts our ability to concentrate, reason and memorise, is a different story.

In the 1950s, people generally spent, on average, eight hours sleeping. In the 21st century, Brits get an average of six hours and 19 minutes of sleep a night, while one in three Australians are not getting enough sleep. Evidently, our attitude towards sleep has changed — and not for the better.

Though sleep deprivation is not treated with the same level of urgency as other public health risks, for Matthew Walker, professor of neuroscience and psychology at the University of California, industrialised societies are caught in the grips of a “catastrophic sleep-loss epidemic” that is slowly killing us.

In this blog, we evaluate the impact that poor sleep can have on cognition — effects that, if left unchecked, will ultimately hinder job productivity and permeate every other aspect of life.

The importance of sleep

It’s impossible to understate the importance of getting enough rest. One-third of our lives are spent asleep. Sleep is a vital physiological process for maintaining homeostasis (the self-regulating processes that happen in a cell or organism to keep conditions optimal), as well as an important mechanism for cell repair. And it’s crucial for a number of brain functions, including how nerve cells (neurons) interact with one another. In other words, sleep replenishes our system. It keeps us healthy.

When a person fails to get enough sleep, all of these critical functions are compromised. Persistent sleep deprivation increases the risk of diabetes, high blood pressure, heart arrhythmia, stroke, obesity, and cancer. And given that poor sleep has been linked to the acceleration of Alzheimer’s, the effects on the brain can be catastrophic, too.

In fact, it’s impossible to identify a cognitive skill that isn’t affected by how much sleep we get. As revealed in a 2017 UCLA study, sleep deprivation disrupts our brain cells’ ability to communicate with one another — leading to mental lapses that negatively affect memory and visual perception.

Without sleep, our cognitive abilities unravel. According to researchers, the effect is even comparable to being drunk. 

The link between sleep and cognition

We only recently discovered that we need to sleep before learning to prepare our brain to take in new information. Without sleep, the memory circuits within the brain become clogged, restricting our ability to absorb new information.

In a revealing experiment, healthy young adults were split into two groups — one of which got a full eight hours of sleep, the other got no sleep at all. The following day, both groups were given a series of new facts to learn and tested to see how effective the learning had been. Without sleep, it was found that the brain was 40% less able to make new memories. This could be the difference between a first-class class degree and a fail.

Not only do we need to sleep before learning, but we also need it afterwards, too. This is far from a new discovery – we knew this back in 1924. Experiments by John Jenkins and Karl Dallenbach indicated that human memory retention improves by 20-40% if we sleep after learning versus the same time being awake. Sleep allows our brains to fully absorb information.

But sleep is more than just knowledge retention. It also drives creativity. A study published in McKinsey Quarterly in February 2016 found that participants who enjoyed a good night’s sleep were twice as likely to discover a hidden shortcut in a task than those who didn’t. During deep REM sleep, the brain fuses memories together to test associations and see if novel new connections can be made. REM sleep causes our brain to create new links, which is why we often wake up with solutions to previously unresolvable problems.

The challenge here is that not all sleep is equal. Our sleep broadly falls into buckets of around 90 minutes. But these buckets are biased towards NREM sleep (memory clearance and protection) at the start and towards REM sleep (creativity and emotional connections) later on. Both the quality and quantity of sleep are equally important.

What happens to your brain if you don’t get enough sleep?

“After sixteen hours of being awake, the brain begins to fail. Humans need more than seven hours of sleep each night to maintain cognitive performance. After ten days of just seven hours of sleep, the brain is as dysfunctional as it would be after going without sleep for twenty-four hours. Three full nights of recovery sleep (i.e., more nights than a weekend) are insufficient to restore performance back to normal levels after a week of short sleeping.”

— Matthew Walker, Why We Sleep: The New Science of Sleep and Dreams

From 1998 to 2002, David Dinges and Hans Van Dongen led a study that monitored dozens of volunteers split into three different groups — some of whom slept four hours, others six hours, and others eight hours — for fourteen days in the lab.

Every two hours during the day, researchers tested their ability to sustain attention by way of a psychomotor vigilance test (PVT), which involved sitting in front of a computer screen for ten minutes and pressing the keyboard’s space bar whenever they saw a flash of numbers at random intervals.

The PVT is a gold standard for measuring sleepiness and reaction time and is routinely employed during the training of pilots and astronauts — occupations where attention and alertness are key. In a PVT, a delay of only half a second suggests a lapse into sleepiness.

The findings of the study? Participants who received a full eight hours of sleep had few attention lapses and experienced no cognitive decline over the two weeks. In contrast, those who had four or six hours’ sleep saw their PVT results decline steadily every day. By day six, a quarter of the six-hour group were even falling asleep at their computer.

But what causes this decline in cognitive performance? Though sleep deprivation is commonplace across the modern world, we're only just beginning to understand its profound effects on the brain and cognition from a scientific perspective.

One region of the brain thought to be affected by sleep deprivation is the prefrontal cortex. This section of the brain’s frontal lobe is involved in alertness, attention, decision making and executive functioning, which describes our ability to organise cognitive processes (e.g. planning ahead, stopping and starting tasks, shifting from one activity to another).

The effects of total sleep deprivation on the prefrontal cortex include a decreased inhibition of response, decision making, divergent thinking and working memory. Sleep loss is also thought to disrupt the connection between the prefrontal cortex and the amygdala — our brain’s emotional control centre. With this in mind, it’s no surprise that insomnia can significantly impair our emotional perception.

Sleep deprivation is also thought to affect the thalamus, a large mass of grey matter that relays motor and sensory signals to the cerebral cortex, and the hippocampus, a small organ located in the brain’s medial temporal lobe associated with memory (particularly long-term memory). In the thalamus, poor sleep is thought to reduce grey matter volume, which is said to be linked to a loss in visual alertness. In the hippocampus, meanwhile, sleep deprivation is thought to reduce neuroplasticity and the proliferation of cells, both of which may impair a person’s memory.

Of course, the adverse physical and cognitive effects of sleep loss depend on a range of variables, meaning they are experienced differently from person to person. For researchers, this makes it difficult to accurately predict and detect an individual’s sleep-deprivation deficits.

According to a 2018 study by the Perelman School of Medicine at the University of Pennsylvania, the key to making such a prediction may be found in small genetic molecules called microRNAs (miRNAs). The study found that microRNAs in the blood are changed by total sleep deprivation for 39 hours and, as viable biomarkers of sleep deprivation, can predict the resulting cognitive performance in adults.

Though further research is needed, microRNAs may one day be used to identify who is most at risk for the adverse effects of sleep deprivation and thus should receive medical assistance to prevent them.

The takeaway

Without sufficient sleep, our brains cannot operate at a normal, healthy level. In the context of work, a sleep-deprived workforce is bad for business. Aside from reducing productivity, sleep deprivation can also increase the risk of accidents for people who operate heavy machinery or carry out manual work. For businesses and their people alike, encouraging people to get enough sleep should be high on the agenda.