Sleep is essential for spring-cleaning our brains, putting our memories into long-term storage and boosting creativity. Getting enough good-quality sleep is vital for our mental and physical well-being.
But how do you know if you are getting enough? According to the National Sleep Foundation, these are the targets you should be hitting at different stages of your life:
|Age||Recommended hours’ sleep|
|1–12 months||14–15 hours|
|1–3 years||12–14 hours|
|3–6 years||10–12 hours|
|7–12 years||10–11 hours|
|12–18 years||8–9 hours|
|18–65 years||7–9 hours|
|65+ years||7–8 hours|
Most people don’t come close to hitting these targets.
Surveys conducted in the UK found that the average adult claimed to get around 6.5 hours a night. Australians do slightly better, hitting an average of seven hours and 18 minutes.
Bear in mind that “sleep” in this context really means “hours in bed”. Since even good sleepers spend around 15% of their time in bed awake, if you are in bed for seven hours you are probably getting less than six hours actual sleep a night. The other thing about taking an “average” figure is that some people need more than average, while others will need less.
So how do you know if you are getting enough sleep? There are a few tests that you can do to see if you are sleep deprived.
The Sleep Onset Latency Test or Spoon Test
The idea behind this test is to see how quickly you fall asleep during the day, if you are given a chance. Day- time sleepiness is a good measure of “sleep debt” and therefore of whether you are getting enough good-quality sleep at night. If you fall asleep while watching TV or at the cinema, then you probably have “sleep debt”.
The great thing about this test is that it doesn’t require any fancy lab equipment; you just need a metal spoon and a metal tray. The version describe below was developed by a famous sleep researcher, Professor Nathaniel Kleitman, from the University of Chicago.
At the weekend, or whenever is most convenient, you skip your usual morning coffee or tea. Then, in the early afternoon, any time between 1pm and 3pm, you go to your bedroom with a metal spoon and a metal tray.
You close the curtains, place the metal tray on the floor by your bed, check the time, then hang your arm over the side of the bed, clutching the spoon. Finally, you close your eyes and try to drift to sleep. The idea is that if you fall asleep, the spoon will drop from your fingers and hit the tray with a loud clang, waking you up. As soon as that happens, you check your watch to see how much time has passed.
- If you fall asleep within five minutes of closing your eyes, it means you are severely sleep deprived.
- Falling asleep within five to ten minutes is deemed to be “troublesome”.
- Falling asleep after 10–15 minutes suggests you have a mild problem.
- If you stay awake for over 15 minutes, you’re probably fine.
An alternative version of this test which is more practical but less fun, is to go to bed in the afternoon, as described, but this time you just set an alarm on your phone to go to after 15 minutes. You then see if you drop off before the alarm goes off.
The Multiple Sleep Latency Test
This is a more sophisticated version of the above, normally carried out in a sleep lab.
When you arrive at the lab, you are attached to numerous machines (to record brainwaves, eye movements, muscle tone, etc.) and asked to lie down in a dark, quiet room during the day. The scientists measure how quickly you fall asleep and how deeply. After 20 minutes you are woken up. Then, two hours later, you do it again. And then again. In fact, you do this a total of five times. This test is used to diagnose whether you have a sleep problem, and if so what type. Do you have narcolepsy or idiopathic hypersomnolence? A breathing disorder or excessive daytime sleepiness? It is expensive, but it is the most reliable way of getting to the root of a persistent sleep problem.
Why do we need to sleep as much as we do?
We know that not getting enough sleep has a big impact on your brain and your body. But why most of us need at least 6–7 hours of sleep each night is more of a mystery.
Horses, giraffes and elephants seem to get by quite happily on a couple of hours, while our fellow primates need considerably more than we do. Orangutans curl up in a bed in the fork of a tree, and get a solid 10 hours, snoring away sweetly like a great, hairy, orange baby. Baboons, on the other hand, sleep on their bottoms while balancing on a branch high above the forest floor. They also sleep for around 10 hours a night, though their sleep is rather more fragmented.
Some anthropologists think that the invention of the bed (or, more accurately, “a sleeping platform”) by great apes, tens of millions of years ago, was a hugely important part of our evolutionary story. Sleeping platforms meant that, unlike the precarious baboons, our remote ancestors could sleep securely in the trees, safe from predators and blood-sucking insects. It also allowed them to get more deep and REM sleep, which presumably boosted their brain power.
But if sleep is so important for brain development, why do humans, with the biggest brains of all the primates, sleep the least? The short answer is: nobody knows.