Introduction
Sleep is a necessary part of our lives, whether we like it or not. At the end of the day, we (hopefully) get the chance to rest our tired minds in some peaceful, well-deserved sleep. As we sleep, we cycle through 4 different stages, together leading to the restful sleep desired.
The Sleep Cycle
Each sleep cycle lasts around 80 to 100 minutes, and consists of 4 stages: aptly named Stage 1, Stage 2, Stage 3, and Stage 4. These are further categorized into two types: REM and NREM. REM sleep is the stage of dreaming, while NREM (not REM) is every other stage.
NREM Sleep
Stage 1. Otherwise known as N1, it is the shortest stage of them all. It normally lasts around 1 to 7 minutes and is basically the transition between wakefulness and sleep. This is a very light sleep, meaning it is the easiest stage to wake someone up. Your heartbeat, breathing, and eye movements slow, and your body starts to relax. Brain activity also begins to slow, and if you remain uninterrupted, you progress to Stage 2.
Stage 2. Here, in N2, light sleep transitions to deeper sleep. When you reach stage 2, you are officially asleep. Your heartbeat and breathing slow even more, and your muscles likewise relax further. Eye movements slow until they stop completely. Brain movements also slow, but occasionally brief bursts of electrical activity take place. Surprisingly, these spikes in activity actually help prevent being woken up.
Stage 3. The last stage of NREM Sleep, known as N3, deep sleep, and slow-wave sleep. This is the period of sleep you need to feel refreshed in the morning, and normally occurs more in the first half of the night (N3 commonly lasts for 20 to 40 minutes during early sleep cycles, but as the night goes on, they shorten). Your heartbeat, breathing, and muscle tone decrease to their lowest point. This stage also includes patterns of brain activity named delta waves.
REM
Stage 4. REM (rapid eye movement) sleep where most dreaming occurs. It occurs about 90 minutes after you fall asleep. Your eyes flutter and brain activity increases, almost to the levels when you are awake. During this time, your body experiences atonia. This is a temporary paralysis of the muscles except for two: the eyes and the muscles that control breathing. Your breathing comes faster and irregularly. Additionally, your heart rate and blood pressure near levels of wakefulness. As the night progresses, REM stages gradually increase in length. Even though the first REM stage may last only a few minutes, later stages can last for around an hour.
References
"Brain Basics: Understanding Sleep." National Institute of Neurological Disorders and Stroke, National Institutes of Health, www.ninds.nih.gov/health-information/public- education/brain-basics/brain-basics-understanding-sleep. Accessed 2 Sept. 2023.
"Sleep Phases and Stages." National Heart, Lung, and Blood Institute, National Institute of Health, www.nhlbi.nih.gov/health/sleep/stages-of-sleep. Accessed 2 Sept. 2023.
Suni, Eric, and Nilong Vyas. "Stages of Sleep: What Happens in a Sleep Cycle." Sleep Foundation, Sleep Doctor Holdings, 8 Aug. 2023, www.sleepfoundation.org/stages-of- sleep. Accessed 2 Sept. 2023.