Stages of Sleep Explained: 6 Factors That Affect Sleep

A sleep cycle refers to the various sleep stages that our bodies cycle through as we sleep. The sleep cycle includes wakefulness (the stage right before you doze off), three NREM sleep stages, and the dream-state of REM sleep. The cycle generally follows this pattern: stage N1, stage N2, stage N3, back to stage N2, and finally REM sleep. A typical sleep cycle takes between one to two hours to complete, and most people will experience four to six cycles during a good night’s rest.

During a sleep cycle, you’ll experience two different types of sleep:

1. 1. NREM sleep: Non-rapid eye movement sleep, also known as non-REM sleep or NREM sleep, is a period of rest that makes up most of the sleep cycle. NREM sleep is composed of three stages that our bodies cycle through as we sleep: stage N1, stage N2, and stage N3.
2. 2. REM sleep: Rapid eye movement (REM) sleep, also known as paradoxical sleep (PS) or desynchronized sleep, is the sleep stage where most dreaming takes place. The characteristics of this sleep type include an elevated heart rate, rapid eye movement, fluctuations in blood pressure, and temporary paralysis in the arms and legs (to prevent you from moving around while you dream).

Here are the different stages of the sleep cycle that you likely experience as you sleep:

1. 1. Wakefulness: Before you fall asleep, you’re in a conscious state of wakefulness, in which your heartbeat and breathing are quicker, and your mind buzzes with electrical activity. You may also briefly return to momentary wakefulness at different times during the night, most often at the end of a sleep cycle, after a phase of REM sleep.
2. 2. NREM stage N1: Also known as the “dozing off” stage, this is the shortest, lightest stage of sleep. Brain activity starts to slow during this stage, but the body isn’t fully relaxed and may experience involuntary twitching. Your heartbeat and breathing also begin to slow, allowing you to transition into the second stage of sleep quickly. However, it’s easiest to wake someone at this stage.
3. 3. NREM stage N2: During this stage, you start to fall into a light sleep. Eye movement stops, your internal temperature drops, and the brain only emits short bursts of activity, known as sleep spindles. During your first cycle of sleep, this stage only lasts about 10 to 30 minutes but increases in length when you reenter the cycle later on 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 where deep sleep occurs. Short-wave sleep, characterized by low frequency and high-amplitude delta wave patterns, is where your most restful sleep occurs. Brain wave activity and blood pressure slow, your body temperature drops, your muscles relax, and your body can recover and repair itself. This stage lasts for shorter periods as the night progresses.
5. 5. REM sleep: After your body drops down 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, your heart rate is elevated, your brain produces active theta waves, your eyes move rapidly and randomly, your blood pressure fluctuates, and your arms and legs may experience temporary paralysis, preventing you from moving around while you dream.

Here are some significant factors that influence a good night’s rest:

1. 1. Light exposure. Your eyes contain specialized light-sensitive cells that detect light and send signals to your brain that tell your body whether it’s day or night. These signals contribute to your body’s internal clock (also known as the circadian rhythm), which regulates the body’s secretion of the hormone melatonin and determines when you’re sleepy and when you feel most awake. With electric light, TV screens, and the glow from smartphones, modern humans are exposed to significantly more light at night, which can disrupt our biological clocks and keep us awake. Learn about how interrupted sleep affects the body.
2. 2. Chemicals. There are a wide variety of chemicals that you may come into contact with on a daily basis that can disrupt your body’s sleep schedule: caffeine, nicotine, alcohol, antihistamines, and prescription medications. Many of these chemicals can change the way your body moves through the sleep cycle—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.
3. 3. Your sleeping environment. The environment of your bedroom can impact how easily your body drifts off into sleep. In general, low light, low to no noise, and a mild temperature (not too hot and not too cold) is the most conducive environment for good sleep. In addition, many sleep pathologists recommend that you reserve your bed for just two things—sleep and sex—rather than for activities like reading or playing video games. That way, your brain will better associate your bed with sleepiness, putting your body into position to drift off for a good night’s sleep.
4. 4. Stress and anxiety. Sleep isn’t purely biological—your restfulness also relies on your mental wellbeing. If you’re especially stressed or have a clinical condition such as anxiety or depression, you’re more likely to have a difficult time falling asleep every night or may wake up several times during the night.
5. 5. Shift work. While many employees work during the day and sleep at night, there are many professions—from airline pilots to medical staff—that often work through the night. If your job requires you to stay up late, your body’s sleep patterns and internal clock will be significantly affected—either by lagging behind and making you sleepy at work or adjusting so dramatically that you’ll have a difficult time sleeping at night in the future.
   
6. 6. Sleep disorders. Many health problems contribute to nighttime sleep deprivation. Sleep apnea is a medical condition in which, during sleep, your body stops receiving oxygen through your upper airway (either due to a blockage or brain signal). People with sleep apnea may wake up choking or gasping for breath between sleep cycles. Narcolepsy is a condition in which you experience extreme drowsiness during waking hours, often characterized by periodic “sleep attacks.” Restless leg syndrome is when you experience irritating sensations in your legs, especially in bed. Insomnia is a condition in which you have difficulty getting to sleep or remaining asleep that can be caused by several factors or other disorders.

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