We all know that it is not a good idea to burn our candle at both ends – even if it does give a lovely light (1).
“If sleep doesn’t serve some vital function, it is the biggest mistake evolution ever made.” Alan Rechtschaffen, sleep research pioneer.
We spend about 1/3 of our lives asleep; so it must be important. But why do we sleep? And, what purpose does it serve?
Thanks to the winners of the 2017 Nobel prize in physiology or medicine – Jeffrey C. Hall, Michael Rosbach, and Michael W. Young (2), we now know a lot more about the mechanisms that control the machinery of our biological clock – our circadian rhythm. Both plant and animal life is determined by the inexorable light/ dark cycle – you could say that we are slaves to the sun. However, on electrifying the night night we have become more and more divorced from the natural circadian clock. Maybe through better understanding the mechanisms involved in sleep – and the importance of sleep – we can break those bad sleep patterns and get the beauty sleep we need.
Jean-Jaques d’Ortous de Mairan demonstrated the existence of the circadian rhythm in the early 18th century; when he showed that a plant will continue the daily cycle of opening and closing its leaves even when placed in his wine cellar. The conclusion was that the cycle is controlled by an endogenous clock that is entrained by the sun. In fact, the sun is also our most important environmental cue or zeitgeber – “time-giver”. Our internal clock runs on a cycle of just over 24 hours – about 11 minutes over (3)- which is reset daily when we draw back the curtains in the morning – it is how the rotation of our planet has been hard-wired into the fabric of our cells. 2017’s Nobel laureates have unraveled the control of this alternating pattern of sleep and wakefulness. The circadian rhythm is controlled by a group of clock genes – two of which: period and timeless; encode for the proteins PER and TIM. PER builds up in during the day and when, numbers are such that it starts to bump into TIM; the two proteins bind and TIM helps shuttle PER into the nucleus where it can inhibit period gene expression – a very elegant negative-feedback loop (4). This small group of genes in turn affect the expression of hundreds, if not thousands, of other genes that exhibit circadian oscillations. And, although the “master clock” may reside in the suprachiasmatic nucleus (SCN) of the hypothalamus, there are molecular timepieces in tissue throughout the body controlling and synchronising sleep patterns, blood pressure, core body temperature, metabolism, and the production of hormones – hormones are like a symphony and your circadian clock is the conductor.
Messing with this rhythm has been linked to Alzheimer’s disease, certain cancers (bowel, prostate, and breast), diabetes, obesity, and poor mental health. During sleep, the space around brain cells expands by 60% (5) allowing cerebrospinal fluid to move through and clean out beta-amyloid peptides (36-43 amino acids) residues that may have built up during the day. These beta-amyloid peptides can go on to form plaques that are predictive of cognitive impairment – even dementia and Alzheimer’s disease (6). And without a good night’s sleep, the body’s production of natural killer (NK) cells drops by 70% – these cells help guard against tumors and fight infection. So, it may not come as a surprise to hear that the World Health Organization have classified shift work as a probable carcinogen due to the disruptive effect it has on the circadian rhythm (7).
As well as physical benefits of sleep, our brains are hard at work hard at work in other areas. During sleep and dreams we lay down memories, consolidate existing ones, and integrate them. The last one may help us to link things in a way we hadn’t previously seen – great for innovation and creativity. It certainly helped Kekulé, whose idea for the ring structure of benzine came to him in a dream. In dreams we also replay bad memories which can help to dissociate the emotional response and alleviate the anxiety bad memories can cause.
When we go to sleep at night – barring candle-burning or children – we sleep in ~90 minute cycles; comprising of 3 stages of NREM sleep (non-rapid eye movement), followed by REM sleep. NREM sleep is when all the memory processing occurs. During stage N2, events such as K-complex and sleep spindles lead to the delta waves of stage N3. It is during this, our deepest sleep, known as slow wave sleep, that hundreds of thousands of your brain cells go through waves of firing and then falling silent 2-3 times per second – in beautiful synchronicity. The sleep cycle then takes us to N2, then N1 before we enter REM sleep. REM sleep is marked by immobilization and rapid eye movement – obviously. It is during this stage that we have our most vivid dreams. To get the full benefit of sleep we need to complete 4 or 5 of these cycles – so, cat-naps aren’t going to do it.
When I was a child, I was taught that a difficult problem is easier to solve if you sleep on it. Similarly, in Spanish we consultar con la almohada (consult the pillow) and in Russian the popular proverb: “morning is wiser than evening” highlights the folly of sending late night emails and tweets.
Night night, sleep tight,
JAM
Editor’s note: [added 24/01/2018] If this little article has piqued your interest, I thoroughly recommend you read Matthew Walker’s “Why We Sleep” – in fact, I think it should be compulsory reading for all.
