Although we spend a third of our lives asleep, in a blissed-out state of inactivity, we hardly ever stop and ask ourselves: “Why do I need sleep?” The only thing we do know is that it makes us feel better, more alert – but we have no idea why? What is sleep’s primary function?

Scientists have spent many years trying to get to the bottom of why we sleep. They have assessed what happens when we don’t sleep. You don’t really want to be trying that one out. Rats die within 2-3 weeks of total sleep deprivation. Scientists have also looked at sleep patterns in different animals to see whether that might shed some light on our needs. While human adults need 7-9 hours of sleep per night, other animals display great variation in sleep patterns, ranging from 22 hours for koala bears to as little as 1.5 hours giraffes. Despite all the research, there is no definitive theory on why we need sleep. Currently, several theories exist and the truth perhaps lies in an amalgamation of them all.

  • Inactivity theory: One of the earliest theories of sleep, also known as the evolutionary theory, the inactivity theory states that sleep, i.e. keeping still and quiet, was a way of keeping animals safe at night during periods of increased vulnerability.
  • Energy conservation theory: This theory claims that sleep’s primary function is to reduce an individual’s energy demand and expenditure in order to conserve energy, which is done in periods of inactivity such as sleep. Competition for and efficient use of energy is extremely important in natural selection. In other words, you will totally lose if you DON’T snooze.
  • Restorative theory: This theory is all about putting back in what we’ve spent during the day. Sleep becomes a time for restoration, rejuvenation and repair. Empirical evidence shows that animals subjected to total sleep deprivation loose immune function and die within weeks. Moreover, many of the body’s restorative functions such as tissue repair, muscle growth and protein synthesis, commonly occur during sleep.
  • Brain clean theory: Maiken Nedergaard, who led a study at the University of Rochester several years ago, claims: “We sleep to clean our brains out.” She discovered that during sleep, brain cells called glia open up unique channels that allow waste products and toxic buildup in the brain – including the same proteins that build up in dementia – to be eliminated. Moreover, scientists have also found that newly formed connections between neurons, which were created during the day, are strengthened and consolidated while we sleep.


That’s what modern science makes of sleep, but just as interesting are the hypotheses on sleep postulated by early thinkers, from Ancient Egyptians through to 17th century philosophers. They believed all sorts of things… Here’s a brief round-up of their shut-eye theories:


Direct line to the gods

In Ancient Egypt, dreams were seen as messages from the gods, so sleeping was in a way like having direct access to the gods. Dreams were analysed by special interpreters and Sleep Temples – hospitals of sorts – existed to treat ailments presumably psychological in nature. The British Library currently holds The Dream Book, a compilation of dream analyses written on papyrus around 1275BC, a mere 3000 odd years before Freud’s Interpretation of Dreams.


Aristotle’s theory

Possibly the first scientific theory of sleep, Aristotle proclaimed in 350BC, “a person awakes from sleep when digestion is complete”. This is of course not true, but we salute him for taking the time to think about why we sleep.


Saucy St Augustine

The 4th century philosopher and theologian St Augustine worried about the fact that some of his dreams were a bit saucy and of a sexual nature. As he was seeking to live a morally perfect and celibate life, his dreams distressed him. In his writings Confessions he asks, “Am I not myself during sleep?” He also distinguishes between “happenings” (that which involuntarily occurs to us during sleep) and “actions” (our waking-state behaviour, based on choice). He concludes that the notion of sin or moral responsibility cannot be applied to our dreams. Brilliant. Thanks to St Augustine we’ve been forever absolved from any inappropriate dreams we might have. Just tell your partner, “But St Augustine says…” when you next wake up in a solo-sweat.


Lucid dreaming

This is the act you really want to be getting in on. Sometimes also called Dream Yoga or Milam, lucid dreaming is a tantric practice and technique where you learn to control your dreams, by maintaining full consciousness whilst slipping into a dream state. It has been taught and practised by Tibetan Buddhist monks since the 8th century but also experienced by teenagers when they stare at the posters of their favourite boy-band for long enough before falling asleep. No joke.


Descartes the dreamer

The 17th century French philosopher, René Descartes, also called ‘the father of modern philosophy’ wrote about sleep and dreams in his seminal text Meditations on First Philosophy. He wanted to find out what he can determine as certainty and thereby claim as knowledge. He famously came up with the phrase ‘cogito ergo sum’ (I think therefore I am), but also with the sentence: ‘How can I know that I am now not dreaming?’ Descartes dream argument says that the act of dreaming gives us evidence that the senses we trust to distinguish reality from illusion should not be fully trusted. Hence any state that is dependent on our senses should at the very least be examined to establish whether it is in fact reality.


Are you really reading this or are you dreaming that you are reading this?

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