Sixty years ago, Ella Fitzgerald sang, “Birds do it, Bees do it.” While she meant falling in love, we can use the line just as well for sleeping. Sleep is an essential aspect of the daily life of all animals, since it offers not only rest and recuperation, but also helps the brain organise itself.
Even fish do it, albeit with their eyes open. Huge ones like the dolphin are reported to sleep for a little over 10 hours daily. Understanding the role of sleep also offers some cues about the evolution of animals, mammals and, ultimately, of us, humans. Dogs sleep for 12-14 hours a day, but are alert even during this period (recall what the Hindu sages advised students: sleep like a dog - light and alert (swana nidra)). Higher up the evolutionary ladder, a bonnet monkey sleeps for 12 hours or more a day, a chimpanzee (closest to us humans) sleeps about 10 hours, while we sleep about 8 hours a day.
Interestingly, while bonnet monkeys sleep wherever, a chimpanzee actually makes a bed (of strewn and collected leaves and torn out branches) on the tree and cozies up to sleep for 10 hours or so. Orangutans do the same, making a new sleeping platform or bed daily, covering themselves with leaves and similar stuff, in order to avoid predators and bloodsucking insects. Dr David R Samson, of Duke University, has carefully collected and compared the sleeping hours and patterns of higher apes, hunter-gatherer humans in the African wild, and modern day humans from the U.S.
He points out to a connection between sleep (both how deep and how long) and the cognitive ability of animals, primates and man. The very fact that orangutans and chimpanzees make a bed, cover it protectively and sleep long and deep itself points to their ability to think, innovate, and arrange to protect themselves. When he monitored their sleep pattern, he found them sleeping deep, with rapid eye movements (REM) in their sleep for only 18% of the time. Now, REM indicates that the brain and body are energised due to dreaming, storing and recapitulating memories, learning and other “cognitive” acts. The rest 82% of the 10-hour-long sleep is deep and restful. Compare this with a bonnet monkey whose REM sleep is only 12% and the remaining 88% is non-rapid eye movement (NREM) sleep.
The percentage of REM sleep across species is roughly correlated with cognition, brain development and activity. This is best illustrated when we monitor human infants. The REM sleep is highest in growing infants (often up to 50%), indicating that their brains are busy collecting, collating, consolidating and using information. With age it gradually drops and becomes about 22% in people over 65 years of age. (Indeed, a similar growth-dependent behaviour in the REM sleep patterns in chimpanzees has been noted by Japanese primatologists).
What about when early humans moved from the phase of hunter-gatherers to organising themselves into groups settled in villages and invented technologies such as making fire, building houses to live in, practising farming and agriculture and so forth? As expected, the sleep-wake patterns also have changed. Dr. Samson and colleagues have been able to compare the sleeping habits of hunter-gatherer tribes in Tanzania (yes, groups such as the Hadzas, exist even today) with those of modern Western humans in the U.S. (Their paper, free access on the web, appears in the American Journal of Physiology and Anthropology, 2017; 1-10). The Hadzas live natural lives, with their daily body clocks (called the circadian rhythm) tuned to the Sun and the Moon as light sources. In contrast, we moderns use artificial light and sleep in closed environments and follow man-made daily work routines. Our body clocks are thus not natural.
As Drs. Samson and Nunn write in an earlier paper ( Evolutionary Anthropology 2015; 24: 225-237; alas, no free access), when they compared the sleep patterns, they found that the Hadzas slept only for about 6.25 hours every night and with a poor quality of sleep (efficiency estimated to be only 69%). In comparison, modern day humans sleep longer (8 hours) and deeper (efficiency 90-94%). The hunter-gatherer sleeps light, alert towards any disturbance such as animal threats, ambient noise, weather fluctuations and other factors – all of which we moderns are free from, thanks to our protected environments.
In a way, these early humans slept the swana nidra- alert and ready, but slept less than we would have thought they did. However, they made up for it by taking a nap every afternoon for about 90 minutes or so. This is something that we moderns need to learn and practise. Thankfully, this habit of an afternoon nap, or siesta as it is called in Spanish, is still in vogue in parts of Europe. The organisation called the Sleep Foundation extols the benefit of an afternoon nap. They say that naps can restore alertness, enhance performance and reduce mistakes and accidents. A study at NASA on sleepy military pilots and astronauts found that a 40-minute nap improved performance by 34% and alertness 100%. If it is good for space-men, it is good for us too!
D. Balasubramanian
dbala@lvpei.org
