The Nature and Function of Sleep

Published on: September 9, 2023 | Reading time: 🕐 5m

Woman sleeping holding a pillow

What is sleep?

Sleep is a period of physical and mental rest, involving a partial or total loss of behavioural control and consciousness. During sleep, there is a decreased ability to react to things going on in the environment, but this doesn’t mean that the body and brain are on vacation. Rather, the brain remains very active throughout the course of the night and many essential functions take place in the body. Let’s take a closer look at what goes on during a night of sleep.

The stages of sleep

Our sleep-wake cycle follows a 24-hour rhythm, also called a “circadian rhythm.” For most people, it’s synchronized with the Earth’s night-day cycle. During each period of sleep, we cycle through two main types of sleep—non-rapid eye movement (NREM) and rapid eye movement (REM) sleep.

NREM sleep is divided into three stages—Stages 1-3, numbered in order of increasing depth of sleep. These stages are characterized, in broad terms, by a reduction of activity throughout the body. Brain waves slow down, blood pressure and body temperature drop, breathing and heart rate decrease, and muscles relax.

NREM Stage 1 (or N1) occurs first and is the lightest and briefest of all stages. During this stage, we drift in and out of sleep in a relaxed state. We are still quite aware of the external environment, so it is very easy to wake up from this stage. Stage 1 is also characterized by the presence of slow eye movements. Whole-body muscle contractions called “hypnic jerks” can also occur. If you’ve ever had the sensation of falling and then jerking yourself awake, it’s probably happened in this stage.

NREM Stage 2 (or N2) is a “light” stage of sleep in which body functions, including brain activity, slow even more and eye movements stop. Occasional bursts of brain electrical activity called “sleep spindles” occur, which have been linked to learning and memory formation.

NREM Stage 3 (or N3) is known as deep “slow-wave” sleep because of the large slow brain waves (called delta waves) that occur during this stage. Other functions of the body also continue to slow down. Stage N3 is the most restorative stage of sleep for the body—muscle and tissue repair is accelerated, growth and development are stimulated, the immune system gets a boost, and energy is replenished. It is also the most difficult stage to wake someone up from, since the body is less responsive to the external environment, and awakenings and arousals are rare. If you are abruptly woken from this stage, you are probably going to be groggy and disoriented. Interestingly, this is the stage from which sleepwalking emerges.

Finally, REM sleep is a distinct state, very different from NREM sleep. It is the stage in which most of our dreaming occurs. Unlike the slow, resting state of NREM sleep, REM is quite active. Our brain waves during REM look quite similar to our brain waves when we’re awake. One of REM’s most distinctive features is the rapid and jerky eye movements that occur. Our eyes dart back and forth, beneath the eyelids, as they—you may have guessed it—follow our dreams. Compared to NREM, not only are our brain waves faster, but our heart rate and blood pressure are higher. Breathing can become fast, irregular, and shallow. Interestingly, however, muscle tone is absent in REM, to the point that the body is essentially paralyzed. This is believed to be a protective mechanism to prevent us from acting out our dreams.

REM has been a bit of a mystery for researchers. Why does the body need this period of sleep that paradoxically looks a lot like wakefulness? Many theories have been proposed over the years, and experts now believe that REM is important for various aspects of mental functioning. For instance, REM and slow-wave sleep appear to work together to consolidate new memories. The part of the brain involved in our emotions (called the amygdala) is very active during REM, leading to the suggestion that it plays a role in processing emotions and connecting emotions to our memories. REM is also thought to be important for brain development and for balancing our brain chemicals and mood.

Sleep architecture

Our sleep “architecture” is the pattern and timing of sleep stages that occur through the night. Sleep architecture is biologically driven and finely calibrated to meet the body’s needs during nightly rest. Over the course of a night, we cycle through the stages of sleep in a predictable pattern. Typically, each sleep “cycle” takes 90-110 minutes, resulting in about 4-5 cycles per sleep period.

A cycle starts in light sleep and progresses through the stages of NREM sleep, from light to deep sleep. Then, it reverses back from deep to light sleep, usually ending with REM. It’s not uncommon to briefly wake up at this point before the next cycle starts.

The amount of time we spend in each stage differs, and it also changes across the night. In total, young adults spend only 2-5% of the night in Stage N1, about 40-60% in Stage N2, about 20-25% in Stage N3, and about 20-25% in REM. The first half of a sleep period consists mostly of Stage N2 and Stage N3, with brief periods of Stage N1 and REM. In the later cycles of sleep, we get more and more REM, with less time spent in Stage N3 (deep, slow-wave sleep).

The sleep architecture we’ve been describing is what’s typically observed for healthy young adults during a normal sleep period. But as we all know, many factors can interrupt our sleep, preventing us from getting a full night’s sleep or cycling through the stages in this typical fashion. So, what happens if one of the stages gets shortchanged? Fortunately, the body is very flexible and can adapt to interruptions. This means that it can recalibrate the intensity and the time spent in the various stages depending on our needs. For instance, if you don’t get enough deep slow-wave sleep one night, deep sleep episodes the following night will likely occur early and be especially deep and long.

Read more: How Much Sleep Do We Need?

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