Sleep Cycle Overview: Explore the 5 Sleep Cycle Stages

The sleep cycle refers to the various stages that the body cycles through as you sleep. This cycle includes wakefulness (right before dozing off), three non-rapid eye movement (NREM) stages (consisting of N1, N2, and N3), and rapid eye movement (or REM) sleep.

The non-REM sleep stages comprise most of the sleep cycle. During these phases, your body rests, but your closed eyes move normally beneath your eyelids. Conversely, in rapid eye movement (REM) sleep, your closed eyes move rapidly beneath your eyelids, which is when most dreaming occurs. During REM sleep, you will also experience an elevated heart rate, fluctuations in blood pressure, and temporary paralysis in your arms and legs.

Not to be confused with the sleep cycle, the sleep-wake cycle refers to the time you spend in sleeping or waking states during a twenty-four-hour period. Maintaining a healthy sleep schedule can keep your memory recall sharp, improve mood, and strengthen the immune system.

A typical sleep cycle takes between one to two hours to complete, and most sleepers will repeat this process four to six times during a good night’s rest. The first sleep cycle of the night is often the shortest, usually lasting between seventy and one hundred minutes, while later cycles last between ninety and 120 minutes on average. Many factors can affect the length of your sleep cycles, including environment, stress, or illness.

The brain cycles through several sleep stages during a night of healthy slumber. Here are the different stages of the sleep cycle that you experience as you sleep.

1. 1. Wakefulness: Before falling asleep, you enter a conscious state of wakefulness. During this stage, your heartbeat and breath quicken, and your mind buzzes with electrical activity. You may also briefly return to momentary wakefulness at different times during the night, typically after a phase of REM sleep.
2. 2. NREM stage N1: Also known as the “dozing off” stage, N1 is the shortest, lightest part of sleep. Brain activity slows down during this stage, but your body isn’t fully relaxed and may twitch involuntarily. Your heartbeat and breathing also slow down, allowing you to transition into the second stage of sleep quickly. However, it’s easier to wake someone at this stage than at any other point in the cycle.
3. 3. NREM stage N2: During this stage, you will fall into a light sleep. Eye movement stops, your body’s internal temperature drops, and the brain emits short bursts of activity known as sleep spindles. During the first sleep cycle of the night, this stage only lasts about ten to thirty minutes, but it gets longer when you reenter it later in the night. Most people spend about half of their sleep time in this phase.
4. 4. NREM stage N3: The third stage of NREM sleep is when deep sleep occurs. Short-wave sleep, characterized by low frequency and high-amplitude delta wave patterns in the brain, affords the most restorative rest. Brain wave activity and blood pressure slow down, your temperature drops, and your muscles relax so that your body can repair itself. This stage lasts for shorter periods as the night progresses.
5. 5. REM sleep: After the body drops into the deepest sleep and slowly rises back up to stage N2, you enter REM sleep, where most dreaming takes place. In REM-stage sleep, the heart rate rises, the brain produces active theta waves, and your eyes move rapidly and randomly. Additionally, the blood pressure fluctuates, and your arms and legs may experience temporary paralysis, preventing excessive movement, which can interrupt dreaming.

Here are the different factors that can affect the quality and amount of sleep you get per night:

1. 1. Chemicals: Various chemicals can disrupt your sleep, including caffeine, nicotine, alcohol, antihistamines, and prescription medications. For instance, caffeine can decrease the amount of time your body spends in slow-wave sleep, while alcohol can increase the number of awakenings you experience during the night.
2. 2. Light exposure: The human eyes contain specialized light-sensitive cells that send signals to the brain to indicate the time of day. These signals help regulate the body’s internal clock (also known as the circadian rhythm) and trigger the secretion of melatonin, which is the hormone that regulates feelings of sleepiness and alertness. With electric light, TV screens, and the glow from smartphones, your body is exposed to a lot of light at night, which can disrupt the circadian rhythm and keep you awake for longer periods of time.
3. 3. Shift work: While many people work during the day and sleep at night, some professions—from airline pilots to medical staff—require employees to stay awake long after the sun goes down. This inverted sleep schedule can affect your sleep patterns and internal clock, either by lagging and making you sleepy at work or adjusting your circadian rhythm so dramatically that you have difficulty sleeping at night.
4. 4. Sleep disorders: Disordered slumber is one of the leading causes of sleep deprivation. Sleep apnea is a medical condition in which the body stops receiving oxygen through the upper airway (either due to a blockage or brain signal) during sleep. People with sleep apnea may wake up choking or gasping for breath between sleep cycles. Narcolepsy is a condition that triggers extreme drowsiness during waking hours, often characterized by periodic “sleep attacks.” People with restless leg syndrome experience irritating sensations in the legs, especially in bed. Insomnia is a condition that makes it difficult to fall asleep or remain asleep.
5. 5. Stress and anxiety: Your mental health can significantly influence your sleep quality. Being stressed or living with conditions such as anxiety or depression increases the likelihood of difficult sleep.
6. 6. Sleeping environment: Your bedroom environment can impact your ease of drifting off into sleep: Low light, little noise, and a mild temperature (not too hot or cold) make the most conducive environment for good sleep. In addition, sleep pathologists recommend that you reserve your bed for just two activities—sleep and sex—rather than reading or playing video games. That way, the brain will better associate your bed with sleepiness, putting the body into the prime position to drift off into a good night’s sleep.

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