Understanding Sleep Stages

Sleep is a complex process that occurs in stages, and we cycle in and out of these stages during the night. The two main components of normal sleep are REM (rapid eye movement) sleep, when the most vivid dreaming occurs, and non-REM sleep (NREM).

NREM sleep consists of several stages, labeled N1 through N3, indicating the depth of sleep. Stage 4 is REM sleep.

Stage 1/N1: This is the “dozing off” period, which typically lasts one to five minutes. The body hasn’t fully relaxed yet, and it’s easy to wake up during this time.

Stage 2/N2: This is when the body enters a more relaxed state; temperature drops and breathing and heart rate slow. Brain activity generally slows as well, although there are short bursts of activity, called sleep spindles. These are believed to help stabilize memory.

Stage 3/N3: This is deep sleep, during which the body relaxes even more. Some experts believe this stage is critical for restorative sleep. Deep sleep typically occurs in the first half of the night, and lasts for 20-40 minutes in each sleep stage cycle.

Stage 4/REM: REM sleep is thought to be essential for cognitive functions such as memory, learning and creativity. Vivid dreams may occur during REM sleep. Generally, a person enters a REM sleep stage after they’ve been asleep for about 90 minutes. REM stages make up about 25% of sleep in adults, with each stage getting longer and longer; the first stage may last a few minutes, while later stages may last for up to an hour.

Each stage of sleep has a characteristic pattern of brain waves that can be seen on a machine called an electroencephalograph (EEG). A full sleep cycle generally takes about 90 to 110 minutes; then, the cycle repeats throughout the night.

Circadian Rhythms and the Biological Clock

Circadian rhythms are the physical, mental and behavioral changes in the body that follow a 24-hour cycle. These processes affect most living things, and respond mainly to light and dark. In fact, sleeping at night and waking during the day is an example of a light-related circadian rhythm, often referred to as the “sleep/wake cycle.” The sleep/wake cycle can affect eating habits, digestion, body temperature, release of hormones, and other vital functions.

The biological clock is a natural timing device that regulates our circadian rhythms. It’s made up of specific molecules that interact with cells throughout the body. Not everyone’s biological clock is wired the same way. For example, we all know people (often dubbed “larks”) who function better and are bursting with energy in the morning, and others (“owls”) who work more efficiently far into the evening. Some of this wiring difference may be innate, and some of it may be programmed. The biological clock can be reset by exposure to light (daylight or artificial), melatonin, or by vigorous activity.

The biological clock switch (also called the “master clock”) is called the suprachiasmatic nucleus (SCN), and it’s located in the area of the brain called the hypothalamus. The SCN receives information from the brain cells about light and darkness and sends signals to other parts of the brain that control hormones, body temperature, and other body functions that influence whether we feel sleepy or wide awake.

Hours before bedtime, the circadian system starts to turn off its alerting (“wake”) signal in preparation for sleep. Typically, the system produces the least alerting signal around 4:30 a.m. Then, even before we wake up, the circadian system switches on its alerting system and our body temperature rises as the body prepares for the day.

A second system that governs sleep is the homeostatic drive — pressure for sleep that builds up in the body the longer you remain awake. The pressure decreases during sleep, reaching a low after a full night of good-quality sleep. During the day, the circadian rhythm’s alerting signal counters the homeostatic drive for sleep, which is why we generally can remain awake.

We disrupt these normal patterns by shift work and, to a lesser extent, when we stay up late and sleep in on the weekends. After changing sleep patterns on the weekend, we may find it difficult to get to sleep on Sunday night, and as a result, you may wake up feeling groggy on Monday.

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