The Science of Sleep Cycles
Our Complex Brainwaves While We Sleep
Sleep Cycle Overview
The science of sleep as we know it was discovered relatively recently when a laboratory at The University of Chicago hooked people’s brains up to electrodes and observed REM sleep in 1953. When we sleep, our body cycles through distinct sleep cycles, also known as sleep stages. There is some debate on how many stages there are, with the American Academy of Sleep Medicine distinguishing 4 stages of sleep (REM, N1, N2, N3) and the European Academy of Sleep Medicine distinguishing 5 stages of sleep (REM, N1, N2, N3, N4). N1 and N2 sleep are thought of as light sleep while N3 and N4 are considered deep sleep. Light sleep and deep sleep are similar in that they are both forms of non-rem sleep, where the body does not have the intense dreams that characterize REM sleep. Many people cycle through these stages throughout the night in the pattern of: Light deep → Deep sleep → Light sleep → REM, but these cycles can vary. It’s common to bounce between light sleep and REM as you sleep throughout the night.
Below are typical brainwaves in the different sleep stages:
In a healthy night of sleep, we cycle through the sleep stages four to five times. It’s common for your body to complete all of these stages in a 90-minute cycle, but this timing can vary. Sometimes our bodies cycle through each stage quickly, whereas other times we linger in certain sleep stages.
We spend the majority of the night in Light sleep. Most wearable algorithms like SleepSpace on the Apple Watch, Oura Ring, and Fitbit combined two stages of sleep to determine light sleep, known as N1 and N2 sleep. Light sleep has more similarities to being awake than the other stages, especially N1 light sleep, in that we can talk, move, and regulate our body temperature while we are in light sleep. A common misconception is that we don’t dream in this stage, but we actually have flashes of dreams. But the dreams in light sleep aren’t the fully formed stories that we usually characterize as dreams. Instead, our .ight sleep dreams are flashes of memories or images that usually don’t form any coherent narrative.
It makes sense that since light sleep is the closest sleep stage to being awake, we are most easily awoken during this stage. If woken during light sleep, you might not even have noticed that you were asleep. If you wake up for less than 5 minutes, you also may not remember that you woke up at all during this stage. It is ideal to wake up during light sleep, especially N1 light sleep, as you will be able to wake up much more easily and feel more alert than waking up in either of the other sleep stages.
N2 light sleep has more regenerative processes than N1 with two unique brainwaves that also characterize this stage. But before we delve much deeper, lets keep in mind that the light and deep stages of sleep follow a gradient and aren’t as clearly defined as you may think. N2 is more often characterized by having certain brainwaves that are thought to be crucial to how we form memories throughout the day and maintain deep regenerative sleep, including Sleep Spindles and K-Complexes.
Defined: Happen in stage 2 NREM sleep and are characterized as bursts in neuron activity between the thalamic reticular nuclei and is designed to help keep us asleep. Sleep Spindle brainwaves play a role in sensory processing, synaptic plasticity, and memory consolidation. They are generated in an ancient part of the brain called the thalamus. If you have more of these brainwaves you are less sensitive to external stimuli. In other words, the more sleep spindles you have, the less likely you are to wake up at night. These bursts of brainwaves follow muscle twitching where the body is thought to be learning about nerve control.
Defined: Occur during stage 2 NREM sleep and are designed to evaluate whether something is from non-threatening external stimuli and to aid in memory consolidation. These brainwaves occur naturally every 1-1.7 minutes and are often followed by sleep spindle. These are brief brainwaves similar in hertz to regenerative delta waves. They tend to generate from the frontal lope, though they occur throughout the cortex.
Deep sleep is the most restorative stage in the sleep cycle. In this stage, our brainwaves become dramatically longer, and our daily experiences are turned into long-term memory. Deep sleep is vital to feeling rejuvenated and learning effectively; without deep sleep, it is difficult to effectively learn and process information. Deep sleep is vital, but it can be difficult to achieve.
As we sleep, we spend less and less time in Deep sleep. In the beginning of the night, it is common to spend up to 50% of the first sleep cycle in Deep sleep. By the end of the night, we only spend around 2% of the sleep cycle in Deep sleep. As we age, our bodies tend to spend less time in Deep sleep per sleep cycle. This is why sleep can feel less rejuvenating the older we get, even if we sleep for the same amount of time.
In REM, like in Light sleep, our brainwaves are similar to when we are awake. Our dreams in REM often form stories that we remember in the morning, as opposed to the fleeting images or memories that characterize Light sleep dreams. During REM our bodies are actually paralyzed, meaning that we are completely motionless except for our eyes. If you see a sleeping person staying very still while their eyes move underneath their eyelids, they’re probably in REM.
When we are in REM, our bodies lose thermoregulation, which means that our bodies cannot regulate our temperatures. In effect, our bodies behave like cold-blooded animals, since our bodies change temperature based on how hot or cold our sleep environment is.