Does Too Much Sleep Have Negative Repercussions?

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Common sense and anecdotal evidence support the notion of consistently getting a good night’s sleep throughout the week. We all know from first-hand experience that sleeping too little makes most of us groggy and prone to brain fog the following day. Lack of sleep wears us down over time. But you've also probably noticed that oversleeping can make you feel discombobulated and like you woke up on the wrong side of the bed, too.

Now, the so-called “World’s Largest Sleep Study,” of 16,812 participants around the globe, has identified an inverted-U association between sleep duration and cognition that illustrates how getting too much or too little sleep both have negative repercussions on brain function. Until now, any empirical evidence associating too much sleep with specific cognitive deficits has been scarce.

This massive online survey of the link between sleep duration and cognition was conducted by neuroscientists from the Brain and Mind Institute at Western University in Ontario, Canada. All of the data for this study was collected using the Cambridge Brain Sciences (CBS) online platform, which has been used by other large-scale studies of cognition in recent years. The latest paper by the Western researchers, “Dissociable Effects of Self-Reported Daily Sleep Duration on High-Level Cognitive Abilities,” was published September 13 in the journal SLEEP.

Getting Between Seven to Eight Hours of Sleep per Night Is Linked to Optimal Cognition

According to the preliminary results of this study, sleeping an average of seven to eight hours per night is linked to performing better on certain cognitive tasks in comparison to sleeping more — or less — than this “Goldilocks” duration of nightly sleep.

Statistically, most teenagers and adults tend to be chronically sleep deprived; oversleeping on a regular basis is not a common concern for most of us. Notably, the researchers found that around half of all study participants reported sleeping less than 6.3 hours most nights. That said, if you are sleeping more than eight hours per night, this research suggests that it could take a toll on your cognitive abilities.

Over 10 years ago, when I wrote “The Sleep Remedy,” chapter for my first book, the prescriptive advice given by myself and most “sleep hygiene” experts was that the body, brain, and mind function best on about 8 hours of sleep per night. The latest advice doesn’t stray far from this time-tested recommendation. The Western researchers also identify just under 8 hours of sleep per 24 hours of awake time as the sweet spot of a “just right” amount of sleep for optimal brain function.

As a simple 2:1 ratio, it seems that for every two hours of being awake, the human body and brain require about one hour of sleep. In a 24-hour day, this means 16 hours of awake time followed by eight hours of sleep.

“We found that the optimum amount of sleep to keep your brain performing its best is seven to eight hours every night and that corresponds to what the doctors will tell you need to keep your body in tip-top shape, as well. We also found that people that slept more than that amount were equally impaired as those who slept too little,” first author Conor Wild said in a statement.

Predicted score performance as a function of sleep duration, for (A) STM, (B) Reasoning, (C) Verbal, and (D) Overall scores, in units of standard deviations. Although STM did not show a significant relationship with typical sleep duration, it is included for comparison. Shaded regions on top and bottom of the curve indicate 95% confidence intervals of the prediction. Vertical dashed lines indicate the location of the curves’ maxima, with shaded 95% confidence intervals prediction (except for STM, where they could not be calculated due to a nonsignificant quadratic term).

Source: Conor J. Wild et al. (2018)/SLEEP journal open access

During the comprehensive online survey, participants filled in details about their sleep habits and then took a battery of 12 different cognitive tests designed to measure three specific domains: (1) Short-term memory or STM (2) Reasoning, and (3) Verbal ability. As you can see in the chart above, reasoning and verbal abilities were more strongly affected by too much or too little sleep than short-term memory performance.

Why Does Too Much Sleep Impair Certain Cognitive Functions?

It makes sense that having a chronic "sleep debt" would impair cognition. However, the reason that too much sleep influences high-level cognitive abilities is still a mystery. As the authors explain:

"While the relationship between too little sleep and cognitive impairment is perhaps unsurprising, it is less clear why too much sleep would produce a similar deficit. A negative effect of nightly sleep in excess of 8 hours is consistent with other similar cross-sectional studies, and is often attributed to known correlates of long sleep duration such as depression, failing health, increased morbidity risk, and decreased physical fitness.

However, we controlled for self-reports of depressive episodes in our analysis, and there is no obvious reason why other such factors would affect specific cognitive domains, and not just overall cognitive performance (i.e., the interaction helps rule out factors that would be expected to affect performance on all tests). A more interesting interpretation is that impaired cognition seen in long sleepers is actually driven by too much sleep; for example, longer sleep is associated with longer and more intense sleep inertia, which has been shown to produce impairments in high-level cognitive domains, like decision making."

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Future analysis by the researchers at Western's Brain and Mind Institute will investigate how cognition varies as a function of sleep duration, when someone wakes up, and the amount of time between waking up and taking the battery of cognitive tests.

Like most large-scale online investigations that rely on questionnaires, this study has some limitations. First and foremost, the authors point out that any cross-sectional approach only takes a snapshot of a participants habits at one moment in time. Therefore, this study cannot establish a causal relationship between sleep duration and cognitive performance; these findings are correlative. Although the researchers did their best to control for obvious confounds like age, gender, education, and levels of anxiety and depression, it’s possible some other unmeasured confounding variable may be driving what appears to be a link between sleep duration and high-level cognitive function. A second potential limitation of this study is that the data was self-reported, which can result in purposeful or unintended misrepresentations. That said, the current results by Wild et al. warrant further investigation and a longitudinal study.

Unfortunately, the average amount of sleep reported in this study was less than seven hours per night. But there is good news: Those who were chronically sleep deprived but were able to sleep between seven to eight hours the night before taking the cognitive tests performed better across the board than fellow insomniacs who didn't get enough shut-eye. This suggests that even if you're suffering from an ongoing sleep deficit, your brain and cognitive function can benefit from a single night of seven to eight hours sleep the following day.

The authors conclude: “These findings have significant real-world implications, because many people, including those in positions of responsibility, operate on very little sleep and hence may suffer from impaired reasoning, problem-solving, and communications skills on a daily basis. The scale of this work paves the way for continuing investigations into how day-to-day sleep patterns, and variations in sleep, affect high-level cognitive functioning in the general population.”