One of the most exciting quotes I enjoyed during my high school History lessons was Napoleon’s remarkable quote about sleep. When Napoleon Bonaparte was asked how many hours of sleep a person should have each night, he replied “six for a man, seven for a woman, and eight for a fool.”
I have always been envious of people who can get by with only six hours' sleep. I prefer eight, sometimes more. Does this mean I am fool? Probably not. I can function on six, but after a few days my brain will be way below full capacity.
How much sleep we get affects us. Too much sleep makes us groggy and disorientated. Too little sleep makes our mood and concentration suffer. If this carries on it can also cause serious health issues, heightening the risk of diabetes, obesity and high blood pressure.
Most of us sleep between six to nine hours a night, meaning we spend about a third of our lives asleep. This may seem like a long time, but we actually sleep the least among all the primates – the group that includes monkeys, apes and us.
So says a new analysis looking at the impact sleep has had on our evolution. The new research suggests that humans have evolved to sleep less, but also to sleep very deeply. This may help explain our success as a species.
Three million years ago, our ancestors still had ape-like bodies. These Australopithecines probably slept in the trees, like modern chimpanzees.
We do not have direct evidence that Homo erectus used fire at this time.
But by two million years ago, hominins had become fully upright. Homo erectus spent its life on the ground, and may have been the first hominin to make beds there. If that is true, we have been sleeping on the ground for a long time.
Sleeping on the ground may have gifted H. erectus with a higher-quality, more restful sleep.
For one thing, they did not have to worry about falling off the trees. What's more, while the risk from predators was higher on the ground, they had ways to protect themselves.
In particular, H. erectus may have mastered the use of fire. The flames and smoke scared away both mosquitoes and larger predators, keeping them safe. They also hide away in sheltered places like caves.
There is some evidence that at the same time hominins came down from the trees, they became smarter and acquired better weapons.
H. erectus was starting to build better tools. Technologies like the Acheulean hand-axes pictured above were more advanced than older stone tools. This knowledge was widely shared, so they must have learned it quickly.
Although we sleep for fewer hours than other primates, the sleep that we have is of high quality so we do not need as much.
To understand whether human sleep is unique, researchers compared the sleep patterns of 21 primates, whose slumber patterns had already been analysed.
Humans therefore have the deepest sleep of any primate.
As well as noting how long the animals slept for, they looked at how much time they spent in rapid-eye movement (REM) sleep. This is when we dream, and when our brain consolidates our memories into long-term storage.
Humans slept the least. The sleepiest primates were grey mouse lemurs and night monkeys, which slept for 15 and 17 hours respectively.
But in contrast, humans spent the highest proportion of their sleep in an REM state: almost 25%. "Humans therefore have the deepest sleep of any primate.
The idea that humans have evolved to sleep briefly but deeply runs counter to a cherished folk belief. Surely modern technologies like artificial lighting have disrupted our natural sleep patterns. Isn't that why we are all so sleep-deprived? Sleep is particularly important when we are very young, especially REM sleep. Infants spend far more time in REM sleep than children or adults.
If you compare a child's REM sleep to that of an adult, the difference is much greater than it is between humans and chimpanzees. This might mean that getting enough quality sleep early on in life was more important in helping our ancestors develop into ever big-brained hominins.
"REM sleep is very important in the development of the nervous system." "That means that cognition in particular is ultimately very reliant on REM sleep."
But the third of our lives we spend doing so is certainly not wasted. Our big brains took millions of years of evolution to get there, so it's only fair that we reward ourselves with a lifetime of adequate rest.