“The sleep paradox: why do humans sleep so little when we need it so much?” [Nature]. The complete article is payrolled, sadly. But it’s an interesting find:
Enter David Samson, a biological anthropologist at the University of Toronto in Canada, asking a provocative question: why, if sleep has so many benefits, do humans as a species sleep so little?
By studying sleep patterns across closely related species, he estimates that humans require roughly 9.5 hours of sleep per day to fulfil their basic biological needs. Yet, averaged across cultures, people get just under seven hours a day2. Samson dubs this 2.5-hour discrepancy the “human sleep paradox” and makes it the central topic of his book, The Sleepless Ape.
To explain it, Samson argues that natural selection favoured short but high-quality sleep when our ancestors shifted from sleeping in trees to sleeping beneath them. Exposure to predators when sleeping on the ground led hominins to condense their rest into shorter, deeper bouts that prioritized restorative rapid-eye-movement (REM) sleep — with the happy by-product of more waking hours for foraging, social interaction and learning to use tools. This idea, which Samson calls the sleep intensity hypothesis, underscores the opportunity costs associated with prolonged sleep. Hume and Locke would have nodded their approval.
I’ve always loved these lines:
Sleep that knits up the raveled sle[e]ve of care,
The death of each day’s life, sore labor’s bath,
Balm of hurt minds, great nature’s second course,
Chief nourisher in life’s feast.
Spoken by Macbeth, who probably isn’t getting even seven hours in….
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Midnight Oil
Cut if you will, with Sleep’s dull knife,
Each day to half its length, my friend,—
The years that Time takes off my life,
He’ll take from off the other end!
First Fig
My candle burns at both ends;
It will not last the night;
But ah, my foes, and oh, my friends—
It gives a lovely light!
Millay