As you sleep, your brain cycles through four stages of sleep. The first three are considered non-rapid eye movement (NREM) sleep, also known as quiet sleep. The fourth is rapid eye movement (REM) sleep, also known as active sleep. Show
In stage 1, you transition from wakefulness to sleep. In stage 2 you enter light sleep. By stage 3 you are in a deep sleep. Stage 4 REM, known as paradoxical sleep, is when the body's immobilized and dreams occur. Each sleep stage has a unique function and role in maintaining your brain's overall cognitive performance. Some stages are also associated with physical repairs that keep you healthy and get you ready for the next day. The entire sleep cycle repeats itself several times a night, with every successive REM stage increasing in duration and depth of sleep. This article discusses the four stages of sleep. It also explains what happens during each sleep stage and what can hinder your sleep. Verywell / JR Bee Falling SleepUsing an electroencephalogram (EEG), a non-invasive test that records brain activity, scientists are able to see how the brain engages in various mental activities as a person falls and is asleep. During the earliest phases of sleep, you are still relatively awake and alert. At this time, the brain produces what are known as beta waves—small and fast brainwaves that mean the brain is active and engaged. As the brain begins to relax and slow down, it lights up with alpha waves. During this transition into deep sleep, you may experience strange and vivid sensations, known as hypnagogic hallucinations. Common examples of this phenomenon include the sensation of falling or of hearing someone call your name. There's also the myoclonic jerk; if you have ever been startled suddenly for seemingly no reason at all, then you have experienced this. Aren't There 5 Stages of Sleep?Sleep used to be divided into five different stages, but this was changed by the American Academy of Sleep Medicine (AASM) in 2007. NREM Stage 1The first stage of the sleep cycle is a transition period between wakefulness and sleep. If you awaken someone during this stage, they might report that they were not really asleep. During stage 1 sleep:
This brief period of sleep lasts for around five to 10 minutes. At this time, the brain is still fairly active and producing high amplitude theta waves, which are slow brainwaves occurring mostly in the brain's frontal lobe. NREM Stage 2According to the American Sleep Foundation, people spend approximately 50% of their total sleep time during NREM stage 2, which lasts for about 20 minutes per cycle. During stage 2 sleep:
The brain also begins to produce bursts of rapid, rhythmic brain wave activity, which are known as sleep spindles. They are thought to be a feature of memory consolidation—when your brain gathers, processes, and filters new memories you acquired the previous day. While this is occurring, your body slows down in preparation for NREM stage 3 sleep and REM sleep—the deep sleep stages when the brain and body repair, restore, and reset for the coming day. NREM Stage 3Deep, slow brain waves known as delta waves begin to emerge during NREM stage 3 sleep—a stage that is also referred to as delta sleep. This is a period of deep sleep where any noises or activity in the environment may fail to wake the sleeping person. Getting enough NREM stage 3 sleep allows you to feel refreshed the next day. During NREM stage 3 sleep:
During this deep sleep stage, your body starts its physical repairs. Meanwhile, your brain consolidates declarative memories—for example, general knowledge, facts or statistics, personal experiences, and other things you have learned. Stage 4: REM SleepWhile your brain is aroused with mental activities during REM sleep, the fourth stage of sleep, your voluntary muscles become immobilized. During REM sleep, your brain's activity most closely resembles its activity during waking hours. However, your body is temporarily paralyzed—a good thing, as it prevents you from acting out your dreams. REM sleep begins approximately 90 minutes after falling asleep. At this time:
Like stage 3, memory consolidation also happens during REM sleep. However, it is thought that REM sleep is when emotions and emotional memories are processed and stored. Your brain also uses this time to cement information into memory, making it an important stage for learning. Repair Work in ProgressDuring deep sleep (stage 3 and REM), your cells repair and rebuild, and hormones are secreted to promote bone and muscle growth. Your body also uses deep sleep to strengthen your immunity so you can fight off illness and infection. Sequence of Sleep StagesIt's important to realize that sleep does not progress through the four stages in perfect sequence. When you have a full night of uninterrupted sleep, the stages progress as follows:
Once REM sleep is over, the body usually returns to NREM stage 2 before beginning the cycle all over again. Time spent in each stage changes throughout the night as the cycle repeats (about four to five times total). Sleep architecture refers to the exact cycles and stages a person experiences in a night. A sleep specialist may show you this information on what's known as a hypnogram—a graph produced by an EEG. How Long Is a Sleep Cycle?A full sleep cycle is generally around 90 to 110 minutes long. What Can Interrupt Your CycleAny time you have trouble falling asleep or staying asleep at night, your sleep cycle will be affected. Interrupted sleep is the term used to describe sleep that is not continuous throughout the night. When this happens, your sleep cycle can be disrupted. An in-progress sleep stage may be cut short and a cycle may repeat before finishing. There are a number of issues that can interrupt your sleep cycles. Depending on which one is at play, this may happen occasionally or on a chronic basis. Some factors that are associated with interrupted sleep and, therefore, may affect your sleep stages include:
What Happens When Sleep Stages Are AlteredNot spending enough time in each sleep stage or properly cycling through the stages of sleep can affect you in a variety of ways, having potentially short-term and long-term consequences. A few examples of issues that can arise from a disrupted sleep cycle include problems with:
People with a disrupted sleep cycle are also at greater risk for:
SummaryAs your body progresses through the four stages of the sleep cycle—stages 1 through 3 (non-rapid eye movement, or NREM) and stage 4 (rapid eye movement, or REM), it transitions through different biological processes that affect your temperature, breathing, cells, and muscles. All the while, your brain is busy forming, organizing, and storing memories. The sleep cycle follows a specific pattern, but that can be interrupted because of variety of habits, health conditions, and even older age. Over time, not getting enough sleep and not cycling through the four stages can cause physical and mental
health issues. A Word From VerywellIt's important not just to get seven to nine hours of sleep per night, but to ensure it's uninterrupted, quality sleep that allows your body to benefit from each of these four stages. If you experience any of the following, make an appointment to see your healthcare provider, as you may not be getting the sleep you need:
Frequently Asked Questions
Which structure is responsible for the passage of materials into and out of the cell Regents?(4) The cell membrane controls the movement of molecules into and out of the cell.
How soaking the fish briefly in salt water before freezing them might prevent this damage to the cells?— Soaking fish briefly in salt water would reduce the water content in the cells, allowing space for the remaining water to expand when it freezes. — The fish cells will lose water, so they won't burst.
Which process allows for the evolution of finches over time?From this one migrant species would come many -- at least 13 species of finch evolving from the single ancestor. This process in which one species gives rise to multiple species that exploit different niches is called adaptive radiation.
Which histogram displays the relative abundance?The horizontal axis represents the relative abundance of SSRs in all analyzed virus genomes.
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