How much sleep is good for you?

The "HEADACHE HURTS" campaign taught us that (especially if you have migraine) about 7 hours of regular, undisturbed sleep is a great way to stop horrible headache attacks or at least not provoke them unnecessarily.

But, on a basic level, how much sleep should you get to stay sharp and feel good all day? Is it possible to sleep too much? How does the amount you sleep affect your well-being?

Conor Wild, a sleep researcher at Western University in Ontario, Canada, found himself unable to rest until he found the answers – and launched a massive study in more than 10,000 people to get to the bottom of it all. Participants were first asked to complete an extensive online form with questions about their sleep patterns, health status, education level and many other variables. The subjects then completed a battery of 12 well-established tests (games, riddles, puzzles, and special association exercises) to assess a range of cognitive abilities. These included reasoning, spatial and short-term memory, verbal fluency, and selective attention. The results of all the assessments were combined to give an overall score summarizing performance across the entire battery of tests.

Participants reported an average sleep duration of 6.4 hours per night over the last month. However, analysis of all the data indicated a sweet spot (optimal sleep amount) of 7.2 to 7.4 hours, depending on how each cognitive ability (thinking, speaking, total index) was weighted. In other words, subjects with that amount of sleep did best in the tests. Longer and shorter sleeps were linked to poorer test results. With one exception: sleep duration had no effect on short-term memory. Also, performance did not differ with respect to gender or age.

Baffling

One result will make you sit up and think: it turns out that one proper night’s sleep can restore you to full performance if you’ve been getting too little sleep. Subjects who slept a good hour longer than their average 6.4 hours for just one night achieved the best results in all the tests. But there was a limit: participants who slept three hours longer than usual experienced a significant drop in performance. So, based on a mean sleep duration of 6.4 hours, performance improved if the actual amount of sleep approached the optimal amount of 7 to 8 hours, even for one night. ‘Big sleepers’ who shortened their night's rest to the optimal duration also improved their importance.

From their observations, the authors developed the following hypothesis: sleeping only six and a half hours a night on a long-term basis leads to a 'sleep debt' associated with a decrease in cognitive abilities. As soon as you pay off that debt, which you can do by approaching the optimal sleep duration (even for just one night), your organism recovers and your brain gets back to peak performance. This means if you sleep too little on a regular basis, you will benefit from a single proper night’s sleep. Conversely, 'normal sleepers' will feel the effects of just a single night with too little sleep.

Save important decisions for when you’ve had a good night’s sleep

The authors say their findings have real-world implications, summed up in the following advice: If you need to make important decisions, get enough sleep the night before. The advice is all the more valuable because many people in positions of responsibility operate on too little sleep, possibly resulting in significantly impaired cognition and communications skills – and a debilitating headache on top.

Like brain, like heart

The brain is not the only organ to feel the effects of getting too little or too much sleep. Studies have shown that the two extremes are cardiovascular risk factors as well. Too little sleep increases your risk of diabetes, hypertriglyceridemia, and high blood pressure. Sleep durations of more than ten hours were also associated with high levels of triglycerides (a type of fat found in your blood) and high blood sugar levels in research subjects. The cause of these metabolic effects is largely unexplained. However, at least for sleep deprivation, there is evidence that it leads to altered levels of certain hormones that regulate appetite, calorie intake and energy expenditure in our organism.

A mysterious exception

So how come too little or too much sleep has no effect on short-term memory, of all things? Evolutionary biology may provide an answer, but this is all complete speculation: perhaps this part of our mind turned out to be crucial for key everyday tasks like finding food, escaping from predators or caring for offspring – and was protected from any interference so that it would be fully available at all times for these crucial activities. In other words, short-term memory’s non-reliance on sleep could be a kind of behavioral relic, a throwback from tribal times when peak intellectual performance was less important than situational flexibility, sort of: now where was that cave I can run to if I meet a wild animal? In these and similar situations, the protective shield around short-term memory may have proved life-saving, which would explain why it has been preserved to this day.  Investigating whether this bold hypothesis has any probability of being true would certainly be an intriguing research topic all of its own.