How Sleep Works in the Brain
The Stages of Sleep
In order to improve sleep, acknowledge the importance of its cognitive benefits on a global level, and promote worldwide health, we must first understand how sleep works in our brain. The first step towards doing so is exploring its basic stages. You usually experience this sleep cycle several times throughout the night, each time with longer durations of certain phases. Here is an overview of the types and phases that make-up a full night’s sleep :
REM vs. NREM Sleep
The two types of sleep are non-rapid eye movement (NREM) and rapid-eye movement (REM) sleep. During NREM sleep, the brain’s activity and heart rate slows down, muscles gradually relax, and body temperatures are lowered, allowing for a period of rest. REM sleep is when your brain becomes highly active and conducts many cognitive processes, similar to wakefulness. During this phase, your heart rate rises and you completely lose control over your muscles.
Stage 1 NREM (N1)
This stage is the transition from wakefulness to sleep, and is usually 1-7 minutes long. The body is not completely relaxed, but heartbeat, brain activity, and eye movements slow down while muscles begin to relax.
Stage 2 NREM (N2)
During this phase, muscles further relax and heart rate and breathing slow down even more. Brain activity decreases but is marked by short eruptions of electrical wave activity that assist the brain in blocking out external stimuli. This phase lasts for about 10-25 minutes in the first sleep cycle, but its duration can extend as you cycle through the sleep stages multiple times.
Stage 3 NREM (N3)
This stage is known as deep sleep, as your heart rate and breathing reach their lowest levels and your muscles are almost completely inactive. Brain waves achieve a particular slow pattern known as delta waves, which is why this phase is also called slow-wave sleep. N3 is known to be vital for many restorative processes, such as tissue repair, bone and muscle growth, and immunity. Research has recently revealed that N3 also contributes to thinking, creativity, and memory. Early in the night, N3 stages are usually about 20-40 minutes long but become shorter as the night continues.
REM
In this stage of sleep, brain activity is at its highest. Most muscles experience a state of paralysis, with the exception of eyes and the muscles that control breathing, known as atonia. REM sleep is crucial for many cognitive functions, such as vivid dreaming, emotional processing, memory consolidation, brain development, learning, and creativity. This phase usually starts about 90 minutes after you have fallen asleep and lasts for a few minutes, but becomes longer in each sleep cycle as the night continues.
Brain Regions and Functions
There are multiple brain regions involved in sleep, many of which contribute to cognitive processes like learning and memory consolidation. Here is an overview of these parts of the brain and their sleep-related functions:
Hypothalamus - a peanut-sized structure located deep within the brain that serves as the “home base” for the circadian rhythm, which is the biological clock of our bodies that regulates our sleep-wake cycle by processing light exposure. It also produces neuropeptides like orexin, which maintains wakefulness, and melanin concentrating hormone (MCH), which promotes REM sleep.
Suprachiasmatic Nucleus (SCN) - a cluster of cells in the hypothalamus that process external light and allows the body to match its biological clock with the light-dark cycle.
Brainstem - the lowest part of the brain consisting of the pons, the medulla, and the midbrain. It controls the transition between sleep and wakefulness by producing the neurotransmitter GABA , which reduces brain activity. It also promotes REM sleep by sending signals to relax the body’s muscles.
Thalamus - a structure located between the brain stem and the cerebral cortex. It relays sensory information to the cortex during REM sleep, such as images and sounds, to create dreams. It remains inactive during NREM sleep to prevent disturbances from external stimuli. It also interacts with the cortex to organize information and promote long-term memory.
Pineal gland - a gland situated within the brain’s two hemispheres that receives signals from the SCN and promotes melatonin production. Melatonin is a neurotransmitter that allows your brain and body to enter the sleeping state.
Basal forebrain - a structure located near the front and bottom of the brain that houses cholinergic neurons, which are regulate of the sleep/wake cycle by producing the neurotransmitter acetylcholine.
Amygdala - an almond-shaped structure that processes emotions. It experiences heightened activity during REM sleep and is involved in the consolidation of emotional memories.
Here is an interactive diagram to help you visualize how these brain structures work together to provide your body with sleep. Click on the labels to learn more!
A diagram describing the main stages of a sleep cycle.
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References
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