From being awake to going to sleep. Now, what happens when you sleep?
A healthy young adult will drift off to sleep after maybe a few tosses and turns. Once asleep, there will be a loss of external awareness, responsiveness, a lowered muscle tone, a loss of conscious sense of time, and a sensory blackout of the brain. Basically, you’re no longer aware of your surroundings.
Scientific verification of sleep, however, is more robust. Electrodes record signals from three different body regions to demonstrate a person’s brainwave activity, eye movement activity, and muscle activity.
When you sleep – both REM and NREM sleep are at work
Sleep can be classified into two phases, being REM and NREM sleep.
Firstly, have you ever watched a person or baby sleeping and noticed their eyes moving from side to side beneath their lids?
This eye movement is called rapid eye movement (REM) sleep, and you know they’re dreaming.
The other phase is when there’s no eye movement and is predictably called non-rapid eye movement (NREM) sleep.
Within NREM sleep, there are four stages. Stages 1 (lightest) & 2 are light NREM sleep, and Stages 3 & 4 (deepest) are deep NREM sleep.
As can be seen in the graph below, NREM sleep lasts longer than REM sleep.
The two phases of sleep, one without eye movement (NREM) and the other with eye movement (REM), last for about 90-minutes and then repeat on a regular cycle throughout the night.
For a blissful 8-hour sleep night, there are five cycles of NREM-REM sleep playing out throughout the night.
As shown in the graph above, the ratio between NREM and REM of each 90-minute cycle is different.
The first half of the night is consumed by deep NREM sleep, with the second part of the night dominated by the dreaming stage of REM sleep, particularly the last 90-minutes of sleep.
Why NREM dominants early sleep and REM prevails in later sleep
Interestingly, NREM is prominent early in our early sleep, while REM prevails in the second half of the night.
The best example I can find to explain the interaction of NREM and REM sleep is an example given by Matthew Walker in his book “Why We Sleep.” He describes it as follows:
Consider the creation of a piece of sculpture from a block of clay. It starts with placing a large amount of raw material onto a pedestal (that entire mass of stored autobiographical memories, new and old, offered up to sleep each night).
Next comes an initial and extensive removal of superfluous matter (long stretches of NREM sleep), after which brief intensification of early details can be made (short REM periods).
Following this first session, the culling hands return for a second round of deep excavation (another long NREM-sleep phase), followed by a little more enhancing of some fine-grained structures that have emerged (slightly more REM sleep). After several more cycles of work, the balance of sculptural need has shifted. All core features have been hewn from the original mass of raw material.
With only the important clay remaining, the work of the sculptor, and the tools required, must shift toward the goal of strengthening the elements and enhancing features of that which remains (a dominant need for the skills of REM sleep, and little work remaining for NREM sleep).“Why We Sleep” by Matthew Walker
This is an excellent explanation of what happens when you sleep and the imbalance of NREM and REM sleep distribution during the night.
What’s the difference between NREM and REM sleep
It’s easy to say the difference between NREM and REM sleep is NREM is responsible for deep sleep while REM is responsible for dreaming. But that’s barely touching the surface.
First of all, what’s common to both NREM and REM is that once you’re asleep, there’s a sensory blackout of the brain. You are no longer conscious of your surroundings.
The sensory signals such as seeing, hearing, touching, tasting, and smelling are blocked by the thalamus, an oval-shaped object, about the size of a small lemon, located in the center of your brain. The thalamus blocks all sensory signals from passing through to your brain’s cortex.
However, the significant difference between NREM and REM sleep is brainwave activity.
The brainwave activity of NREM and REM sleep
A characteristic of deep NREM sleep (stages 3 and 4) is brainwave activity, which can be described as “slow-wave sleep.” The brainwaves are characterized as deep, slow, and synchronized (rippling from front to back of brain).
In deep NREM sleep, there’s a dramatic reduction of the up and down tempo. It’s about ten times slower than the speed of your brain activity while awake.
During NREM sleep, there are additional bursts of brain activity called sleep spindles. These sleep spindles are essential in memory consolidation and play other roles, like blocking external noises (sensory gating).
With REM sleep, your brainwave activity is shallow, rapid, and chaotic (similar to when you’re awake), as you can see in the diagram below.
Memory consolidation of NREM and REM sleep
NREM sleep’s deep sleep brainwaves are generated in the middle of your frontal lobes and travel from the front of your brain to the back.
What’s important is that these traveling brainwaves will move recent experiences (memory packets) from short-term storage locations to more permanent long-term storage sites of the brain. In summary, deep NREM slow-wave sleep encourages information transfer and the distillation of memories.
On the other hand, REM sleep is associated with dreaming that takes new memories and interplays them with your entire back catalog of memories. During this period, various signals, such as emotions, motivations, and memories (past and present), are played out in the cortex of our brain’s cortex.
The dream state of REM sleep is responsible for interconnecting our past experiences. In doing so, an ever more accurate model of how the world works is created for us, including problem-solving abilities and creative insights.
Something else happens during REM sleep. There’s no muscle tension or tone in the muscles of your body. You are entirely limp. Once REM sleep is finished, muscle tone returns.
From an evolutionary and practical perspective, the lack of muscle activity during REM sleep is ultra important in stopping us from acting out often very bizarre movement-driven dreams.
A concluding note on NREM and REM sleep
All sleep is essential, and both NREM and REM occur during all 5-cycles. But each serves a different function.
The first half of the night is dominated by NREM sleep that transfers newly learned information to long-term storage sites within the brain.
In the second half of the night, REM sleep dominates, particularly in the final hour before waking from an 8-hour sleep.
REM sleep takes our newly acquired memories and makes associations with all our previous memories and experiences. Indeed, it’s REM sleep that fuels our creativity and helps forge links between unrelated pieces of information.
Yes, all sleep is essential, but the last hour of sleep is super important as daylight approaches.