What is sleep?

You sleep in two basic phases, which repeat themselves several times nightly; Rapid Eye Movement (REM) sleep and Non-REM sleep. You start with NREM sleep, which Involves falling asleep and eventually reaching deep sleep. In NREM sleep your brain waves become bigger and slower, which is known as Alpha and Theta activity. These morph into Delta waves, deep slow wave activity where the regenerative work of sleep really happens – the body heals and regenerates itself, releasing human growth hormone and strengthening the immune system. In REM sleep, which starts about 90 minutes after you have fallen asleep,  your brain becomes much more active and your brain wave activity is faster and smaller. Beta activity as it is known is the same brain wave activity as when you are awake, even though you are asleep. REM sleep lasts 20 – 30minutes and comes 4 or 5 times a night.

REM sleep seems to be associated with memory consolidation and learning, During REM sleep the parts of your brain responsible for emotions are particularly active, the hippocampus and the amygdala. The hippocampus is involved in turning short-term memories into long-term ones, which may help explain why many times we dream about things that were relevant to us recently. It also helps us in making associations, between things which initially seem not to be connected. For example, a chronic pain sufferer who injured his knee at work had recurring dreams of running. Prior to his injury this man was always in a hurry and his initial thought when he got injured was that he could just “walk It off.” As the months went by and his leg failed to improve, he was not moving at all; just sitting in a corner at home and not talking to anyone. The running dream represented his wish to escape his situation, something he had always been able to do in the past. The amygdala on the other hand, is highly involved in fear and aggression. This could explain the violent nature of some dreams, or you can imagine an overactive amygdala leading to nightmares.

In contrast, the part of your brain responsible for thinking, (the mPFC) is normally quiet during sleep. As you can imagine, the mPFC needs to rest and recover functioning for the next day.  The mPFC and the amygdala are connected and need to work in harmony with each other for good sleep. For example, the amygdala needs the mPFC to be quiet so it can process and make new memory associations. As you are about to learn, this does not always happen in insomnia.

The mPFC is also part of a brain system called the Default Mode Network which is responsible for daydreaming and planning and preparing. The DMn is kind of lie your worry circuit. The DMN is always on except when you are busy with tasks that require focused attention, and/or asleep. So in terms of conscious mental activity its on when you are off and off when you are on. An overactive DMN can make you feel more anxious, stressed and depressed.

Falling asleep is triggered by a part of your brain located in your brain-stem called the Pons. The Pons is responsible for arousal, physiological responses to arousal, and things like heart-rate,  sleep, respiration, emotional arousal etc. The Pons is also responsible for the orienting response, your brains ability to detect and respond to sensory stimuli. By controlling arousal the Pons makes sleep possible.

One of the main ways it does this is through the production of Neurotransmitters such as Norepinephrine, Dopamine and Seratonin, responsible for arousal, pleasure and mood and sleep respectively. Norepeniphrine is a stress hormone, part of the fight or flight response. Its purpose is to mobilize the body for action. Norepeniphrine levels are lowest at 3 AM and gradually increase throughout the day until around 3 PM (Ziegler et al. 1976). Dopamine is a feel-good chemical which should decrease during sleep but actually increases in insomniacs. In addition to its role with mood, Seratonin is also necessary for the production of melatonin, a hormone which is very important or sleep.

How much sleep is normal?

In the modern era 7 – 8 hours of continuous sleep per night is considered normal. However before the invention of the electric light many people slept in two separate segments of roughly four hours. They would go to sleep at sunset and sleep for four hour, and then Wake up in the middle of the night for a couple of hours which might be spent talking, having a hot drink, praying or meditating, before going back to sleep for another four hours until dawn. This bi-phasic sleep pattern. was considered to be “a normal part of life’s rhythms,”

In pre-industrial times Human sleep also followed a more natural day-night cycle. Without artificial light There was no escaping the dark of night which was a time of mystery where people were forced people to take a break from their daytime activities and preoccupations.

In pre-industrial times people also didn’t worry about how much sleep they got  so much because they measured time differently. They were not subject to the measured, deadline-driven allocation of time that we are. Time was a much more flexible resource marked by the changing of the seasons. So If you didn’t sleep well and didn’t feel up to planting that crop in the morning It didn’t really matter –tomorrow was another day.

Summary

• While we still have a lot to learn we can say some things about sleep;
• Sleep is necessary for rest, memory processing and learning.
• Sleep is also necessary for the brain to clean out toxins.
• Sleep helps the immune system.
• During sleep there is a loss of self-conscious awareness.
• Some regions of the brain are less active, others are more active.

Now that we have some understanding of what happens in the brain during normal sleep, we have some basics for understanding what goes wrong in the brain during stress-related insomnia, which we will look at in the next article.

References

Marques, DR, Gomes AA, Clemente V et al  (2015) Default-mode network activity and its role in comprehension and management of psychophysiological insomnia; a new perspective. New Ideas in Psychology 36, 30-37.

Salas, RE (2014 ), Galea JM, Gamaldo AA et al . Increased Use-Dependent Plasticity in Chronic Insomnia. Sleep Mar. 1:37(3):535-44.