The Science Behind Sleep

What are the multiple roles of sleep. What happens in your body and brain while you sleep? What are the 4 cycles of sleep?

Credit: Metamorworks

The Definition of Sleep

Day after day, we go to bed and wake up the following morning. We don’t usually put much effort into it; it’s natural, just like eating or drinking water.

But have you ever wondered what sleep is, anyway, and why do we sleep when we have so many other things to do with our time?

Scientists define sleep as a reversible state of inactivity associated with reduced responsiveness to weak external stimuli.

Why Do We Sleep?

First Studies and Observations

We still don’t know for sure why we sleep. Scientists have conducted experiments using lab animals to understand this better.

They found that every animal examined sleep (or at least exhibit a pattern of rest and activity that resembles sleep). During sleep, animals don’t eat, drink water, or reproduce and are more vulnerable to predators. Even so, this behavior persisted through evolution. The fact that sleep was conserved despite negative selective pressures shows it must have provided an evolutionary advantage.

One of the ways to study sleep function is to restrict or deprive animals of sleep. Researchers tried this with fruitflies. They forced them to keep moving during their sleep period and observed an increase in immobility during the day when they are typically active.

Therefore, there’s a “force” telling the organism to sleep. This is called sleep pressure and is present in other animals, including humans. The pressure gets stronger the longer we’re awake and decreases during sleep.

Functions of Sleep

Sleep is essential for our overall health and well-being. As we discussed, the exact function of sleep remains an enigma. Sleep likely had a primary function when it originated, and the evolutionary process added more functions later. Scientists believe sleep serves multiple functions instead of a single one.

Below, we’ll explore some of the potential functions of sleep.

Brain development

Sleep might play a role in the development of the brain and its neurological pathways. REM sleep may help form new nerve cells. However, more studies are needed to conclude.

Sleep and circadian rhythms may also influence synaptic plasticity, which are chances that occur at the junctions between neurons that allow them to communicate.

Mental Health

Sleep and mental health are closely linked. Sleep disturbances may increase the risk of developing mental health issues such as anxiety and depression. At the same time, people who live with mental health disorders are more likely to experience trouble sleeping.

Also, many studies demonstrate that treating sleep disturbances positively affects mental health.

Memory Consolidation

Research suggests that good quality sleep helps consolidate declarative memories (i.e., personal information and general knowledge). Other studies found no improvement in remembering motor sequences (i.e., playing piano).

Thus, the effect of sleep on memory can differ depending on the type of memory.

Metabolism

During sleep, your metabolic rate slows down, and your body temperature drops. This is your body's way of compensating for energy expended while awake.

The reduced energy need might result from muscle loss of tone during sleep, as active muscles consume a large amount of energy.

Sleep would be necessary to keep this low-demand state and allow the energy to be restored and the metabolism to work correctly when the individual awakens.

Immune System

An infection can make you tired, and doctors often recommend sleeping to help you recover faster.

On the other hand, poor sleep quality weakens your body’s defenses, making you more vulnerable to infectious diseases.

These observations demonstrate that sleep affects immune system parameters.

For example, studies show that good-quality sleep helps to increase antibody counts following immunization.

How Sleep Works

Anatomy and Physiology of Sleep

Two fundamental processes regulate the sleep-wake cycle: the circadian and the homeostatic.

Circadian rhythms are physical, mental, and behavioral changes that follow a 24 hour-cycle. Your circadian rhythms make you sleepy at night and more alert during the day.

Circadian rhythms are driven by the suprachiasmatic nucleus (SCN) of the hypothalamus.

The homeostatic process refers to a sleep need that accumulates during the day. The longer you stay awake, the stronger the sleep drive gets. This is why we tend to sleep longer and more deeply after sleep deprivation.

In addition to the SCN, other brain areas are essential in regulating the sleep-wake cycle:

• Brain stem – it communicates with the hypothalamus and controls the transition between wake and sleep. It’s also important to promote muscle atonia (loss of muscle tone) and help initiate REM sleep.

• Thalamus – it prevents sensory signals from reaching the cortex, letting you tune out the external world. However, this area becomes active during REM sleep when we’re dreaming.

• Pineal gland – it receives signals from the SCN to produce melatonin, a hormone that induces sleep.

• Amygdala – this area is involved in processing emotions and is active during REM sleep.

Sleep Phases

Sleep has two phases: Rapid eye movement (REM) and Non-rapid eye movement (NREM sleep)

Each phase is linked to specific brain waves, variations in muscle tone, and eye movement. During a typical night, your body cycles through all the stages of non-REM and REM sleep. A night can have 4 to 6 cycles; each cycle takes approximately 90 minutes. We spend roughly 75% of the time in the NREM stages and 25% in REM sleep

The Four Stages of Sleep

Stage 1 (N1)

In the first sleep stage, you transition from wakefulness to sleep. The electroencephalogram (EEG), a test that measures electrical brain activity, detects mainly alpha waves (8-13 Hz). This brain wave pattern is associated with relaxation and drowsiness. Later, these waves are replaced by theta waves (6-8 Hz). Your muscles start to relax, and your heart and breathing rate slow down. This lightest sleep can last 1 to 5 minutes, consisting of 5% of your total sleep time.

Stage 2 (N2)

In this stage, sleep becomes deeper compared to stage 1. The EEG shows sleep spindles and K-complexes. Sleep spindles are brief, powerful bursts of neuronal firing in specific brain areas. They may play a role in memory consolidation. K-complexes are long delta waves (1-4 Hz) that last about 1 second. They are also associated with memory consolidation. Your heart rate and body temperature drop. Your muscles relax even further, and your eye movement stops. This stage lasts around 25 minutes. The first cycle is the shortest, becoming longer with each successive cycle. Overall, you spend 45% of the time in N2 sleep.

Stage 3 (N3)

N3 is also known as slow-wave sleep (SWS). This is the deepest sleep, and it becomes more difficult to wake up while you’re in this stage of sleep. EEG recording shows signals with lower frequencies and higher amplitudes, known as delta waves. While sleeping, you spend approximately 25% of the time in N3 sleep.

REM sleep

The EEG during REM sleep is similar to what’s observed when awake. However, your skeletal muscles become paralyzed. This helps prevent you from acting out your dreams.

Your eyes move rapidly from side to side while you keep your eyelids closed. This is why this sleep stage is called rapid eye movement (REM) sleep.

Your breathing rate becomes faster and more irregular. Your heart rate and blood pressure increase to near-waking levels. It’s also during REM sleep that most of the dreaming activity occurs.

The first REM episode occurs about 90 minutes after you fall asleep and typically lasts 10 minutes. REM becomes increasingly longer and deeper as the morning approaches. The last episode can last one hour.

Summary

Sleep is critical for our health and well-being. Research suggests that sleep plays multiple roles. For example, it participates in brain development, keeps our minds healthy, promotes learning, consolidates memory, and helps our body store energy and fight diseases. Understanding the science behind sleep can help us prioritize its value in our daily routine to maintain optimal health.

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