Sleep: Why is it Necessary?
Sleep is important as it enables us to integrate new information, associate and connect with past experiences, enabling insight and problem-solving.
At the moment there is plenty of time for sleep. Roughly a third of our time is spent sleeping and the neuroscience behind it has always fascinated me. Social distancing means of course staying at home, avoiding people, cancelling any plans which may be ideal for an introvert.
The song ‘There ain’t nuthin’ goin’ on but the rent’ was released by the American singer Gwen Guthrie in 1986 in reference to a popular urban cliché of the early 1970’s and it made her famous. As my list of jobs at home shrinks, I am inclined to agree that I don’t have much to talk about! There again, perhaps for many small businesses unable to trade I am sure that the last thing they want to talk about is rent.
So, I am going to write about all the bits in between our daily monotony of social distancing; that is our sleep.
The Need for Sleep
I remember reading an article in the National Geographic in 2017 which had amazing photos from the Indian Ocean of a group of 30 adult sperm whales sleeping motionless in a vertical position 15 metres below the sea surface. Apparently, they nap for 10-15 minutes in this position and research suggests that they might be one of the worlds least sleep-dependent mammals.
In general, how much different species sleep is largely dictated by how much time they spend eating. This is certainly true for herbivores such as large grazers like giraffes who will only sleep 5-30 minutes a day. As ruminants they have a diet that is of low calorific density and requires a great deal of processing. Meanwhile apex predators such as lions, are at the other end of the spectrum. A male lion may sleep for 20 hours or so, while a female lion may sleep for 15-18 hours (please resist the urge to comment ladies).
So, given the fact that we all spend a good part of our lives sleeping, what is the purpose?
In 1611 Shakespeare wrote in Macbeth act two, scene two that sleep ‘is the chief nourisher in life’s feast’. He wasn’t wrong. Fundamentally without sleep we cannot learn or create new memories from the many novel experiences we are exposed to each day. Many functions of the brain are also restored by sleep and obviously concentration and our focus would be very much impaired without it.
With the advent of recent technologies such as MRI and CT scan it has become increasingly evident that our brains and bodies are remarkably active as we slumber. Much of our sensory information is processed through a part of our brain called the thalamus. Essentially this is a sensory convergence zone and it acts as a gateway allowing privileged information to travel through to our cortex for conscious perception. When sleep is initiated the thalamus blocks incoming sensory information and we lose contact and perception of the outside world as we enter the phases of deep sleep.
Whilst we are blissfully unaware of any sense of time in the outside world, our brain physiology is meanwhile running metronomically with an impeccable sense of precision like a Swiss watch. So what is it doing?
REM and NREM
Traditionally sleep has been catalogued using three basic indicators. These are brainwave activity, eye movement and muscle activity which are then presented as a polysomnograph. This same equipment was used in 1952 by Professor Nathaniel Kleitman and Eugene Aserinsky who first discovered that we enjoyed two phases of sleep: one with eye movement (they termed this rapid eye movement or REM) and one without (termed as non-rapid eye movement or NREM). Oddly they also observed that brain activity during REM sleep was comparative to when we are awake. This is connected of course to the experience of dreaming. Variously as time moved on NREM was categorised into a further four stages of increasingly deeper sleep.
Since these early times it has also been established that these two phases of sleep cycle every 90 minutes starting with a descent into NREM followed by an ascent into REM, as shown above in what is known as a hypnogram. Different animals interestingly have different sleep cycles. Oddly insects and fish, unlike birds and mammals, do not experience any REM. Returning to our giraffes, they on the other hand, like horses and cows, lie down for short stints for REM sleep. Meanwhile aquatic mammals and some birds utilize something called unihemispheric slow-wave sleep. This allows them to shut one side of the brain so they can continue to function. This is an evolutionary ideal for the purposes of long migrations or maintaining functions if you are in marine a environment.
What is also very clear in the hypnogram is, as we progress through the night and into morning REM begins to dominate. This is another interesting point as getting up very early in the morning potentially means that although you lose a small percentage of your overall sleep you lose 60-90% of your REM sleep. The same is true if you go to bed late when you lose a high percentage of your NREM time.
Our Brainwave Patterns Asleep and Awake
Another important indicator of the different phases of sleep are our brainwaves. When we are awake our brainwaves are cycling 30-40 times per second in what is known as a ‘fast frequency’ pattern that is irregular and erratic. Clearly this is because the brain is busy processing a variety of inputs (sound, vision, smells and emotions) which when combined have no discernible pattern.
When we fall into NREM in stages 3 and 4 our brain waves decelerate to become slow waves as low as 2-4 waves per second. These slow waves resonate from the thalamus from the front of our brains sweeping through all neural tissue as the wave travels from front to back. The brain was originally considered to be idle during this phase, but we now know that this is one of the most stunning examples of neural collaboration. The brain enters a unified state in electrical harmony.
What essentially happens during this process is that this allows information transfer between neural anatomy. It is a bit like a desktop computer filing information into different directories and folders. This allows us to store or distil experiences and integrate them into short or long-term memory. This helps store and reinforce new facts and skills.
When we undergo REM sleep our brainwaves are not dissimilar to when we are awake with faster frequency brainwaves (REM is accordingly sometimes known as paradoxical sleep).
Bearing in mind we are not conscious of any external stimulus in our environment during REM why are the patterns then similar? Obviously, the brain is busy doing something. REM sleep it would appear helps all our newly filed information to integrate, associate and connect with past experiences enabling insight and problem solving.
Another unique aspect of REM is our muscle tension. During an NREM cycle muscle tone in key postural muscles decreases marginally but conversely during REM they become totally lax. This prevents us from living out our movement rich sleep experiences. A key exception is obviously our eyes which is how REM was discovered in the first place.
The Connection between Sleep and Memory
To give you an idea of how important the role of sleep is in absorbing novel information a simple study was performed on a group of adults by Matthew Walker and described in his book ‘Why We Sleep’. These people were divided into two groups and asked to memorise 100 different faces paired with names throughout the day: quite a taxing process. One group was allowed a 90-minute siesta and the other group stayed awake. When the subjects were tested later in the day those that had a siesta performed significantly better than the awake group, in fact there was a 20% difference in scores.
Interestingly a small finger-like projection in the brain called the hippocampus is largely responsible for short-term memory storage, but it has only a limited capacity. Matthew Walker’s studies analysed the electrical activity of the brains of subjects while they had their siestas and he noticed short bursts of powerful activity in the second phase of NREM (these bursts of activity are called spindles).
As it turned out, the more spindles that occurred the greater the restoration of learning capacity. At the same time, they also recorded electrical activity between the hippocampus and the long-term memory storage sites of the cortex. This meant that these people had a refreshed learning capacity when they awoke as new information was filed into longer term memory thus effectively emptying the hippocampus.