Despite advances in neuroscience and medicine, sleep remains one of the most mysterious functions of the brain. We still do not fully understand why it is necessary, exactly how it is regulated, or even how it evolved in the first place. After all, the idea of “switching off” for hours in the wild – where something might be trying to eat you – hardly sounds like a good survival strategy from an evolutionary perspective.
And yet sleep exists in almost all animals. That alone suggests its benefits must be substantial enough to outweigh the risks and can be considered the most fundamental evidence of its biological importance. We still struggle to fully define the reasons why it is the case, but research continues to bring us closer to an answer. What we do know with certainty is this: sleep is a complex, actively regulated process, even if it appears outwardly inactive.
Why Is Sleep an Active Process?
It is precisely this outward inactivity that leads us to mistakenly perceive sleep as a completely passive state. Intuitively, we extend that association and assume that nothing at all is happening in the body or brain. We might compare it to turning off the lights: once the switch is flipped, activity at every level fades away. With almost no visible movement, it is easy to imagine that everything inside is “quiet” as well.
But that turns out to be far from the truth. To see what is really happening, we need additional tools. For example, to observe brain activity, we can use electroencephalography (EEG). This method represents brain activity as waves that vary depending on the brain’s state. Wakefulness has its own characteristic EEG pattern – a recognizable way these waves look and interact. Sleep, it turns out, has its own distinct pattern as well. The “silence” we imagined earlier is not there. The brain continues to work, but the way it works changes fundamentally.
If wakefulness resembles a busy daytime office juggling dozens of parallel tasks, sleep is more like the night shift: less hustle, but plenty of essential, highly organized work.
What Is Sleep Made Of?
Sleep is organized in cycles. Over roughly 90 minutes, the brain completes one cycle consisting of two phases:
• Non–REM sleep (NREM) – slow–wave sleep, or deep sleep
• REM sleep – rapid eye movement sleep, also known as paradoxical sleep
NREM Sleep: A Descent Into Depth
Each night, sleep begins with NREM sleep. That may seem counterintuitive, since the onset of sleep does not feel particularly “deep.” But if we observe brain activity on EEG, we see that it is not an abrupt drop. It is a gradual descent — like moving step by step down a staircase. The brain passes through lighter stages before reaching truly deep sleep.
At this stage:
• Brain activity slows and becomes more steady and synchronized.
• Heart rate and breathing decrease.
• The body relaxes, though it remains capable of movement (likely to allow repositioning during sleep).
• Body temperature drops slightly
Continuing the comparison: if wakefulness is the daytime office and sleep is the night shift, then during this phase the brain seems to say, “The daytime rush is over. It’s time for maintenance.”
And that is essentially what happens. This slowing of activity creates the conditions needed to clear away the byproducts that accumulated in the brain during wakefulness. At the same time, other important processes take place — processes necessary for the healthy functioning of multiple systems in the body. For example, several important mechanisms for maintaining an adequate immune response are active during this phase. We will not explore this in detail here, but it is worth remembering that the scale and diversity of what occurs can be compared to citywide cleanup and preparation before a major event.
REM Sleep: The Cinema of Dreams
What follows slow and steady deep sleep? One might assume an even deeper state. In reality, something far more intriguing occurs. If we continue observing EEG activity, at some point the brain appears to “wake up.” The slow waves of NREM sleep are replaced by faster patterns that closely resemble wakefulness. And yet this is REM sleep. This similarity to wakefulness is what gave it the name “paradoxical sleep.” This resemblance is not accidental. Some of the mechanisms that sustain REM sleep also play a role in maintaining wakefulness.
During this phase, several important and fascinating processes unfold: we dream, the brain engages in complex processes related to emotional regulation and processes essential for learning and memory. It may seem as though the brain “wakes up just a little” to act as its own psychologist, to watch a dream like a movie, and sort the events of the day into memories. But only “a little,” because several systems are actively suppressed during REM sleep (otherwise we would fully awaken). Activity in regions responsible for stress responses, executive planning, and behavioral control is decreased. The ability to produce voluntary movements is also effectively inhibited, the brain literally “switches off” most muscles. This likely serves as a protective mechanism. Without it, we might literally attempt to run from the tiger that the brain has so convincingly imagined in a dream. One notable exception to this muscular “shutdown” is the eye muscles. During this phase, they perform rapid movements — which is exactly what the name REM refers to: rapid eye movement.
At this stage:
• In contrast to NREM sleep, brain activity accelerates and becomes more irregular, resembling wakefulness.
• Heart rate and breathing increase.
• Voluntary movement is impossible; the body’s muscles are largely “switched off.”
• Body temperature can be slightly unstable
Putting it all together, a typical night looks roughly like this:
• We gradually fall asleep, passing through lighter and deeper stages of NREM sleep – steady state, with slow brain activity
• The brain then transitions into REM sleep – dream-filled, with activity resembling the wakefulness
• This NREM-to-REM progression forms a cycle lasting about 90 minutes. As the night progresses, the proportion of time spent in each phase changes – toward morning, REM sleep occupies a larger share of each cycle.
• Eventually, wake-promoting systems regain dominance, and instead of entering another sleep cycle, we awaken. Ideally, feeling refreshed and energized
Why Is It Important to Remember That Sleep Is Active?
Sleep quality affects:
• Concentration and performance
• Emotional stability
• Learning and memory
• Overall health
Its contribution is comparable to proper nutrition and physical activity—and sometimes even exceeds them. If we view sleep as mere inactivity, it becomes easy to sacrifice. But when we understand that the brain is actively working throughout the night to sustain sleep, our attitude may shift. It becomes easier to make thoughtful decisions to protect it.
Understanding sleep as an active process also helps explain why certain substances can disrupt it. Take alcohol as an example. Alcohol can slow reaction time, impair coordination, and reduce concentration—effects that result from disruption of specific neural mechanisms. The same disruption affects mechanisms essential for maintaining sleep. The brain struggles to sustain stable sleep; it becomes more fragmented, overall quality declines, and REM sleep is particularly affected.
The drowsiness caused by alcohol’s sedative effect acts as a deceptive trap. It may seem that falling asleep quickly guarantees good sleep—but we now understand that sleep is far more complex than simply “switching off.” Quality sleep depends not only on how quickly we fall asleep, but also on maintaining appropriate physiological conditions throughout the night.
The same applies to chronic sleep restriction. If we shorten sleep duration, the brain simply does not have enough time to complete the necessary number of cycles required for full physiological restoration. So, we should remember that each night our brain works actively on our behalf, sustaining sleep. Let us protect that effort, to ensure that “refreshed and energized” truly described our mornings.
In the next article, we will discuss what constitutes high-quality sleep and what sleep hygiene really means.