In search of sleep's sweet spot: Is too much a bad thing?

The world of sleep has continually been a tough one to crack - our dominant waking hours have often made our bedrooms an afterthought, leading to a general lack, lower quality and gross misunderstanding of our nightly snooze. The consequences and costs of inadequate sleep, especially when experienced by so many people, go far beyond just affecting the individual sleeper. It's been estimated that sleep deprivation costs Canada $21.4 billion in productivity each year. Coupled with the personal health risks of inadequate sleep (everything from a greater risk of obesity to Alzheimer's), it's clear that we've crossed the threshold into a global concern in almost every regard.

While we all are well aware that regular, quality sleep is beneficial for our moods, minds and bodies, continued research is revealing that reaping those benefits is more nuanced than we dreamed. According to new findings in the world's largest sleep study, getting too much sleep could be just as detrimental as getting too little.

The study, from neuroscientists at Western University, gathered a sample of over 10,000 participants from 173 countries, aged 18 to 100, mainly residing in Canada, the United States, the UK and Portugal. In addition to divulging information on their lifestyles and sleeping habits, participants were asked to complete a series of 12 tests, which evaluated their attention, inhibition, reasoning, memory, planning and cognitive flexibility (the ability to think about two or more different concepts). Researchers then compared their sleep times (over the past month and the night before the tests) with their test scores.

Participants slept an average of 6.42 hours per night in the month leading up to the test and, on average, 6.88 hours the night before the test - both below the recommended 7-8 hours of shuteye. Perhaps unsurprisingly, those who slept less than the recommended amount performed poorly on the tests. However, those who slept more than 7-8 hours also performed similarly, suggesting that, as far as cognition is concerned, getting too much sleep might be just as detrimental as getting too little. The cognitive areas that were most affected by either end of the sleeping range were decision making and verbal skills, while short term memory remained largely unchanged. Of those who fell outside of the optimal sleep duration, the more than half were under-sleeping. Amongst those under-sleepers, those who slept the least saw the most dire consequences - participants who regularly slept around 4 hours or less per night showed the cognitive equivalent of aging nearly 8 years.

While there is a naturally cognitive decline with age, it would be a logical assumption that the study's older participants would show a greater cognitive sensitivity to not getting the optimal amount of sleep, but that was not the case. Participants of all ages were affected equally by improper sleep in their cognitive performances, suggesting that even the seemingly resistant youth are not immune to these consequences. Furthermore, the fact that older participants who were getting the recommended shuteye had better test scores also debunks the myth that you require less sleep as you age.

Though this may all seem like bad news for chronically poor sleepers, the study also discovered that regular under-sleepers who slept for the optimal 7-8 hours the night before the testing performed noticeably better than those who hadn't. So, while you might not necessarily be able to catch up on "sleep that you missed", it's encouraging to know that one night of quality sleep can make a difference.

Researchers are clear on why sleeping too little causes cognitive impairment (since adequate sleep is integral for the brain), but less so on the effects of too much sleep. One hypothesis they offer is that longer sleep duration could cause longer and more intense sleep inertia - that groggy transition from sleep to full wakefulness - and they suggest more studies should head in this direction.

Hopefully, as researchers continue to dig deeper into the hows and whys of our sleeping hours and put sleep myths to the test, we can continue to approach it in a more nuanced and strategic manner, to hit that "sweet spot" of shuteye (in both duration and quality) and take maximum advantage of what it offers us.