The Science of Sleep Cycles

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.

Light Sleep

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 light 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, let's 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.

Sleep Spindles

Sleep Spindles are smaller and faster waves that typically happen in stage 2 NREM sleep. They are characterized as bursts in neuron activity at around 12-15 Hz 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. Studies have shown that individuals who have more sleep spindles during the night perform better on tests that require motor skills and procedural memory the next day.

K-complex

K-complexes are large, slow waves that often 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. They can be triggered by a variety of stimuli, such as noises, smells, or even internal bodily sensations. K-complexes are thought to serve as a protective mechanism, helping the brain to screen out irrelevant stimuli and prevent us from waking up unnecessarily.

So, what is the relationship between K-complexes and sleep spindles?

Recent research has suggested that they may work together to promote overall brain health. When a K-complex occurs, it triggers a brief period of silence in the brain, during which no other brain waves are present. This silence may be necessary for the brain to consolidate memories and process information without any interference. Sleep spindles, on the other hand, are thought to help protect the brain from external stimuli during this silent period. They may also play a role in transferring information from short-term memory to long-term memory, a process that is essential for learning and retaining new information.

Deep Sleep (also known as Slow-Wave-Sleep)

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. Learn how deep sleep stimulation can increase your deep sleep.

Rapid Eye Movement REM Sleep

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. Below is a graph of changes in sleep stages throughout the human life-cycle from age 5-85.

The Importance of Each Stage of Sleep

Both NREM and REM sleep are critical for overall health and well-being. For example, studies have shown that people who get enough deep NREM sleep have a lower risk of developing obesity, diabetes, and cardiovascular disease. REM sleep has also been linked to improved mental health, including a decreased risk of depression and anxiety.

Additionally, the timing of sleep cycles is important. Our bodies are programmed to follow a circadian rhythm, which is a 24-hour internal clock that regulates our sleep-wake cycle. Disrupting this rhythm, for example, by staying up late or traveling across time zones, can lead to sleep disturbances and negative health effects.

How to Optimize Sleep Cycles

To optimize sleep cycles, it's essential to establish healthy sleep habits. This includes:

• 1) Consistently getting 7-9 hours of sleep per night

• 2) Going to bed and waking up at the same time each day

• 3) Creating a relaxing bedtime routine

• 4) Avoiding caffeine and alcohol before bed. Its best to stop caffeine consumption by around 2 PM for most people.

• 5) Keeping a cool, dark, and quiet sleep environment

In addition, tracking sleep patterns can provide valuable insights into the quality of rest. Wearable devices, such as smartwatches or smartrings, can monitor sleep stages and provide feedback on how to improve sleep hygiene. In order to estimate sleep stages, heart data is necessary. However, an electroencephalogram (EEG) provides the gold standard measurement of sleep stages when used, known as polysomnography in sleep science.

Dr. Gartenberg has conducted research to better understand the relationship between K-complexes, sleep spindles, and delta waves and the quality of sleep. He has developed technology that uses sound stimulation to enhance the occurrence of sleep spindles and improve overall sleep quality. This technology works by detecting the presence of slow-wave brain activity and then emitting an auditory cue that is timed to occur during the peak of a sleep spindle, thus reinforcing the spindle and potentially enhancing its effects on memory consolidation and cognitive function. This technology is known as deep sleep stimulation.