In this article, we will be focusing on explaining light sleep which represents stages 1 and 2 out of the three non-REM (NREM) stages. In some ways, light sleep can be seen as a preparation phase for deep sleep and REM sleep, and it’s much easier to wake someone in the lighter stages. Let’s look into the details.
By now, surely you’ve heard of sleep stages. It may be hard to imagine and visualize that concept at first, as sleeping people tend to all look the same to an untrained eye. However, the use of scientific diagnostic tools like the electroencephalogram (or EEG, for short) helped us understand that sleep is a much more complex process than what was originally thought, with a lot more brain activity than it would first appear. We started to understand how the restorative processes of the body are focused around specific stages of sleep, and how various disorders and external factors can upset the delicate balance of these stages and create health risks further down the line.
In this article, we will be focusing on explaining light sleep. It represents stages 1 and 2 out of the three non-REM (NREM) stages we’re aware of. We used to separate NREM sleep into four stages, but recent research developments and consensus have “combined” stage 3 and 4 into one. Light sleep makes up about 50 percent of adult sleep time, although it can get longer depending on age and other factors. In some ways, light sleep can be seen as a preparation phase for deep sleep and REM sleep, and it’s much easier to wake someone in the lighter stages. Let’s look into the details.
It’s important to understand how a regular sleep cycle works. Light sleep is hard to analyze on its own, as its connections to other sleep stages determine its functionality and properties. A healthy body is capable of calculating the ideal sleep architecture for itself, which is why medication side-effects that extend a specific sleep stage or cut it short can easily compromise your health. Let’s look at and summarize the four distinct stages of sleep that form each sleep cycle.
Stage 1 only lasts a short time (around 5-15 minutes usually, taking up about 3% of your overall sleeping time) and doesn’t repeat during other cycles. During this stage, you’re transitioning from a waking state to a sleeping one. As your body prepares for stage 3 (deep sleep), your breathing and heart rate go down, as does your body temperature. You become less responsive to any external stimuli as your consciousness slowly drifts away. At some point (it can be quite hard to tell) you enter stage 2. Your eyes make slow rolling movements, as you’re not very far into your first sleep cycle.
Stage 2 is the main topic of this article. Your body temperature drops further in this stage, and your breathing and heart rate follow suit. Eye movement stops, and your brainwaves slow down significantly. This stage occupies roughly 50% of your sleeping time, although the cycle distribution may vary, as REM sleep becomes more and more prevalent over the course of a night. While it’s not stage 1, it’s still fairly easy to wake you up in this stage. This stage of light sleep comes with signature brainwave patterns, called spindles and K-complexes. We will cover them in more detail later on in the article.
Stage 3 is simply called deep sleep. During this time in your sleep cycle, you are almost completely disconnected from the stimuli in your immediate environment, making you much more difficult to wake up. Your muscle movement relaxes considerably, and your breathing and heart rate slow down even more – you barely move at all in stage 3. Dreams do not occur in this stage, but it is still quite important for healing your body. Your organs and blood are cleansed of toxins, and your tissues are rebuilt. In regular sleeping conditions, stage 3 occupies around 20% of total sleep time.
Stage 4 is REM sleep. Another name for REM sleep is “paradoxical sleep,” as the brainwaves and certain other processes within the body resemble what a waking person would display on the readings. Certain parts of the brain are even more active than when you’re awake, but others are dormant. This stage is where memories are consolidated, and your mind is “sharpened” for the coming day. Part of the reason people with sleeping problems don’t perform well mentally during the day is that they don’t get enough REM sleep. Initially, it takes around 80-100 minutes to enter this stage, and from then on it’s a regular part of your cycles, becoming longer and longer until you wake up.
Light sleep can sometimes be considered the “filler” stage as it will last as long as it needs to while deep and REM sleep aren’t happening. It doesn’t mean its functions are trivial or irrelevant. During light sleep, the brain transfers memories from short-term storage to long-term storage. Think of it as making a “memory backup,” much like you would with important files on your computer. As a result, stage 2 sleep is crucial for trying to learn anything. It is achieved through the activity of sleep spindles.
Sleep spindles are at the core of why light sleep is so beneficial to our mind. They are specific brainwave patterns that define stage 2 of our sleep cycle. The name spindle comes from how the brainwave looks when you examine the EEG reading. It is a quick burst of oscillatory activity that is widely theorized to represent brain arranging or experiencing a transfer of electrical energy. Not every spindle will appear the same on an EEG reading. They can differ in location and frequency, as well as their association with slower waves. The main potential reason is that spindles occur as a result of different processes in the brain, although we don’t have enough research to determine which processes those are. Aside from memory consolidation, these spindles are widely associated with cortical development. The amount of spindle activity (or sigma power, as spindles are also known as sigma waves) has been correlated to an individual’s intelligence, specifically their performance IQ.
Spindles can occur at almost any part of the brain, but any given spindle is localized to the part of the brain where it pops up. They appear more often as light sleep is initiated or right before it finishes, and the frequencies can depend on which section of the brain receives the activity. The frontal part of the brain produces slower frequencies (around 10-13Hz) than the centroparietal part (13-15Hz).
Without getting into further detail, it’s pretty clear why the hypotheses about spindles have formed. If light sleep is responsible for memory consolidation and learning, these brain waves could very likely be the direct cause or method. But there’s one more brainwave that defines light sleep, and if you’ve been experiencing disorders like restless legs syndrome or obstructive sleep apnea, it may be especially relevant.
K-complexes are almost the opposite of spindles when you look at the EEG reading. Instead of being rapid bursts of oscillation, they’re slower, larger in their fluctuations, and serve a different purpose. They react to external stimuli that usually come from the room or even further outside. A spindle typically follows a k-complex brainwave, and this is interpreted as the brain exerting effort to keep the person asleep. Anomalies and disruptions in how these waves form are closely linked to disorders such as epilepsy, obstructive sleep apnea, and restless leg syndrome.
According to some recent research, the frequency of spindle production (meaning how often you experience them, not their specific frequency on the EEG reading) affects how difficult it is to wake you up during stage 2. While light sleep takes its name from the idea that it’s very easy to rouse someone from this state, there are variations from person to person. The more often you experience spindles, the harder it is for you to be disturbed while sleeping, which leads many people to believe that spindle production frequency is directly linked to sleep quality overall.
In general, light sleep is the only state that doesn’t have a recommended minimum duration. It is very rare for an adult or teenage person to spend less time in stage 2 than is necessary. Because light sleep has to be “cleared” first before you can enter deep and REM sleep, it’s much more likely that a person will spend an excessive amount of time in stage 2. Sleep disorders and environmental factors that may wake the sleeper up shift the balance drastically in favor of light sleep, denying the person crucial restorative processes that characterize deep and REM sleep.
Depending on a given person’s age, they can have vastly different EEG readings during sleep, mostly related to the activity of spindles and k-complex brainwaves. Sometimes light sleep is barely involved at all. For example, babies spend very little if any time in light sleep. Sleep spindles that define this stage don’t develop at all until around 6-8 weeks after birth. K-complex waves take much longer to develop properly, first showing up at around five months of age.
Meanwhile, older adults tend to spend most of their sleeping time in light sleep. They often deal with an issue called fragmented sleep. As a result, they keep waking up in the middle of the night, and get an insufficient amount of deep and REM sleep, ruining their sleep architecture entirely. Deep sleep is the main sleep stage when it comes to your body repairing itself and improving the immune system. REM sleep is the primary stage for mental repairs such as memory consolidation, and it’s also where your mental clarity and sharpness get restored for the following day. As a result of this dynamic, older adults who spend a vast majority of time in the light stages tend to feel adverse effects on their body and mind, as the crucial restorative portion of sleep never occurs for them, or is drastically less efficient. It is theorized (and research is slowly confirming these theories) that this dysfunctional sleep structure directly leads to problems like Alzheimer’s or dementia.
We want to quickly clear up a possible misconception here. When a person is called a “light sleeper,” this doesn’t mean they’re currently in stage 2 or that they experience more light sleep than usual. Instead, people refer to how easily you can wake someone up. A light sleeper is someone who is more susceptible to being disturbed and awakened by external stimuli, thanks to their lower spindle production frequency. A deep or heavy sleeper is the opposite, being harder to rouse and having a higher spindle production frequency. While we can define light and deep sleep by how easily someone can be woken up, these traits still vary from person to person and change with age. Because a senior sleeper spends more time in light stages on average, they may be easier to wake up even if they were originally a heavy sleeper.
Deciding how long you want to nap doesn’t have to be tied to your daily work schedule only. Depending on how much fatigue you’re experiencing and whether you’re looking to rest after an intense study session, you can optimize your nap time to achieve the best results. As a rule, the longer you sleep, the harder it is to feel truly energized when you wake up from your nap. However, a slightly longer nap (around 90 minutes at most) is excellent for helping your newfound knowledge settle in. Conversely, if you’re only looking to get rid of some fatigue, a quick (15-20 minutes or less) nap is a better choice. Light sleep is all you need to have a refreshing nap.