Stages of Sleep Explained: How Brain Hormones Control Your Sleep

Sleep is a state of decreased responsiveness to stimuli and reduced activity that can be easily reversed. Although we are unconscious during this activity, sleep is an important means for our bodies to recuperate, conserve energy, and form memories. There are four stages of sleep, which can be grouped into two broad categories: rapid eye movement (REM) and non-REM sleep. Non-REM sleep is subdivided into three stages. All four stages of sleep reoccur throughout the duration of the sleep cycle and each fulfills important functions.

 

Key Takeaways: Stages of Sleep

• Sleep is comprised of four stages (N1, N2, N3 and REM) in two categories (non-REM and REM sleep).
• The hypothalamus is the major driving force for wakefulness and sleep through circadian rhythm and sleep homeostasis.
• Non-REM sleep is a progression from the lightest to the deepest stage of sleep with slowing of heart rate, blood pressure, respiratory rate, and muscle activity.
• REM sleep is a more active stage of sleep associated with dreams and faster brain wave activity.
• Tips for improving your sleep include setting a schedule, avoiding stimulating activities near bedtime, performing relaxation techniques before bed, and exercising regularly during the day.

Your Brain While You Sleep

Several parts of the brain are involved in sleep and wakefulness. These areas include the thalamus, hypothalamus, basal forebrain, pineal gland, and portions of the brainstem. When a person sleeps is determined by the interaction of their body’s circadian rhythm and sleep homeostasis. The circadian rhythm acts as the body’s internal clock and is controlled by circadian oscillator cells in the suprachiasmatic nucleus of the hypothalamus. It roughly corresponds to a 24 hour cycle, although it is affected by external light and temperature. Sleep homeostasis, or sleep drive, determines the length of wakefulness and the intensity of sleep when it occurs. Although the exact mechanism controlling this drive is poorly understood, it is thought to be related to the release of the neurotransmitters GABA and adenosine from the basal forebrain and other surrounding areas.

 

Illustration of the body clock. Depending on sunlight perceived by the eye, signals are sent to the suprachiasmatic nucleus, where the circadian clock is housed, in the hypothalamus, which controls various biorhythms. ACOPIN / BSIP / Getty Images Plus

Although the thalamus, basal forebrain, and brainstem promote wakefulness through release of stimulatory hormones, the hypothalamus in particular drives the awake state with hormones such as glutamate, histamine, and orexin. However, as the time for sleep approaches based on the circadian rhythm and sleep drive, these hypothalamic signals decrease to allow sleep to begin. The central autonomic system and the pineal gland are also influenced by the sleep drive to appropriately affect the sympathetic and parasympathetic nervous systems for the changes in bodily functions seen during the stages of sleep.

 

Non-REM Sleep

Stage 2 Non-REM Sleep. Numbering the traces from top to bottom, 1 & 2 are electroencephalograms (EEG) of brain activity; 3 is an electrooculogram (EOG) of movement in the right eye; 4 an EOG of the left eye; 5 is an electrocardiogram (ECG) trace of heart activity. 6 & 7 are electromyograms (EMG) of activity in the laryngeal (6) and neck (7) muscles. James Holmes / Science Photo Library / Getty Images Plus

Non-REM sleep begins the sleep cycle and can be divided into three stages of sleep based on body functions and brain wave activity. These stages are called N1, N2, and N3 sleep based on order of occurrence.

 

N1 Sleep

This first stage of sleep is the lightest period. N1 only lasts for up to 10 minutes and is the easiest stage to arouse a person. During this period, the body begins to relax by decreasing its heart rate, blood pressure, eye movements, and respiratory rate. Muscle activity also decreases, however, twitches can often be observed. If brain activity is measured using an electroencephalogram (EEG), the pattern will show slowing of alpha waves, the rhythmic patterns typically seen while awake, as well as lower voltage waves.

 

N2 Sleep

This second sleep stage continues body relaxation with further decreases in heart rate, blood pressure, respiratory rate, and muscle activity. However, eye movements have stopped by this phase. In order to further decrease energy requirements, body temperature also drops by about 1 to 2 degrees F. An EEG performed during this stage will often show K waves, or long, high voltage waves lasting up to 1 second, and sleep spindles, or periods of low voltage and high frequency spikes, as the telltale signs. Overall, this stage lasts about 10 to 25 minutes.

 

N3 Sleep

The final period of non-REM sleep, N3, is the deepest stage of sleep and the most difficult to awaken a person. Bodily functions relax to their slowest point during this phase. Muscle activity continues to decrease, but slight movements are still possible in this stage. The typical EEG shows a high voltage, slow and irregular pattern known as delta waves. N3 lasts for about 20 to 40 minutes before transitioning to the REM sleep stage.

 

Rapid Eye Movement (REM) Sleep

REM Sleep. Numbering the traces from top to bottom, 1 & 2 are electroencephalograms (EEG) of brain activity; 3 is an electrooculogram (EOG) of movement in the right eye; 4 an EOG of the left eye; 5 is an electrocardiogram (ECG) trace of heart activity. 6 & 7 are electromyograms (EMG) of activity in the laryngeal (6) and neck (7) muscles. James Holmes / Science Photo Library / Getty Images Plus

REM sleep is a departure from the other stages of sleep, as the brain becomes more active during this phase almost to the degree or even more so than while awake. This increase in activity is often associated with vivid dreaming that occurs in this stage. As the name implies, the eyes move rapidly sideways during this period of sleep. Heart rate, respiratory rate, and blood pressure begin to increase closer to their values while awake. However, limb muscle activities are temporarily paralyzed.

 

EEGs obtained during REM show sleep patterns with low voltage and fast waves, some alpha waves, and muscle twitch spikes associated with transmitted rapid eye movement. REM sleep is the longest period of the sleep cycle and lasts for 70 to 120 minutes. As the duration of sleep progresses, the sleep cycle favors increased time spent in REM sleep. The combination of recurring non-REM and REM sleep is hypothesized to enhance the body’s physical and mental rest as well as aid in memory formation.

How to Improve Your Habits for Healthier Sleep

The amount of sleep required per day varies depending on a person’s age and ranges from up to 18 hours for infants, 9-10 hours for school-age children, to 7-9 hours for adults. Although sleep has drastic importance for our health and well-being, most people don’t receive the recommended requirement per night. A 2016 CDC study found more than 33% of adult Americans received less than 7 hours of sleep a night. Lack of sleep has been associated with higher risks of chronic diseases such as high blood pressure, diabetes, obesity, heart disease, stroke, and mental illness.

Tips or rules for better sleep and healthy sleeping habits. SiberianArt / Getty Images Plus

In order to improve your sleep habits, you can follow these tips recommended by the NIH. First, follow a regular schedule for daily bedtime. This step will help set your body’s circadian rhythm and make it easier to fall asleep at a regular time. Second, avoid stimulating activities close to bedtime, including those with bright lights and sounds or with exposure to screens. Similarly, avoiding consumption of caffeinated or alcoholic beverages close to bedtime is generally recommended, as these increase hormones that stimulate the body. Finally, exercise for 20 to 30 minutes daily but not fewer than a few hours before bedtime to further promote healthy circadian rhythm patterns.

Sources

• “1 In 3 Adults Don't Get Enough Sleep.” Centers for Disease Control and Prevention, 16 Feb. 2016, https://www.cdc.gov/media/releases/2016/p0215-enough-sleep.html.
• “Brain Basics: Understanding Sleep.” National Institute of Neurological Disorders and Stroke, U.S. Department of Health and Human Services, 13 Aug. 2019, https://www.ninds.nih.gov/Disorders/Patient-Caregiver-Education/Understanding-Sleep#2.
• Moore, Robert Y. “Clinical Update - Circadian Rhythms, Hypothalamus, and Regulation of the Sleep-Wake Cycle.” Medscape, https://www.medscape.org/viewarticle/491041.
• “Natural Patterns of Sleep.” Natural Patterns of Sleep | Healthy Sleep, 18 Dec. 2007, http://healthysleep.med.harvard.edu/healthy/science/what/sleep-patterns-rem-nrem.
• “The Characteristics of Sleep.” The Characteristics of Sleep | Healthy Sleep, 18 Dec. 2007, http://healthysleep.med.harvard.edu/healthy/science/what/characteristics.