What Happens When You Sleep? Sleep Stages Explained

During an average night of sleep, you cycle through four different stages and two types of sleep. As you transition into sleep, your breathing, brain waves, and heart rate slow, and your muscles relax. Then, as you sink deeper into sleep, your body temperature lowers and your metabolism slows. Various processes happen in your body to promote cell regeneration and memory retention.

When you sleep, your circadian rhythm is also at play. Also known as your internal body clock, the circadian rhythm is directly tied to the suprachiasmatic nucleus (SCN) in the brain’s hypothalamus, which coordinates the timing of biological events—like hormone release, body temperature, sleep, and digestion. Your circadian rhythm dictates sleeping patterns, blood pressure, blood sugar, melatonin levels, and your ability to stay awake during the day. When your circadian rhythm is consistent, you can expect to fall asleep at roughly the same time and get the same amount of sleep every night.

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

• 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 your body cycles through as you sleep: stage N1, stage N2, and stage N3.
• 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).

Neuroscientists have determined that the human body goes through four stages of sleep.

1. 1. NREM stage N1: This first stage of sleep contains the transition from wakefulness to sleep. Common behavior during this stage includes drowsiness, muscle relaxation, mild twitching, and slow eye movements. Alcohol use can trigger this stage of sleep, but it does not facilitate the more restful forms of deep sleep.
2. 2. NREM stage N2: Better known as light sleep, this sleep stage occurs the most frequently during a full night of sleep. During this stage, your body temperature drops, eye movements stop, and brain waves slow. 3.__ 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 in the brain, 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.
3. 3. 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. Characterized by rapid eye movement, this stage features intense brain activity. During this sleep stage, your breathing and heartbeat increase, but your muscles undergo paralysis to prevent you from physically acting out the events of your dreams. Your heart rate decreases as you cycle back into non-REM sleep.

In addition to the processes listed below, consistent sleep can help protect your body against hypertension, high blood pressure, heart disease, anxiety, and depression.

• Sleep helps you process emotions. When you achieve rapid-eye-movement sleep (REM sleep)—the sleep stage in which you experience dreams—the parts of your brain that oversee thinking, memory, and emotions are active. Getting enough sleep each night helps you properly process that information and maintain your mental and emotional health.
• Sleep helps with memory consolidation and learning. Deep sleep allows you to conserve energy for the next day by lowering your body temperature, blood pressure, and heart rate. This energy conservation promotes increased glucose metabolism, which helps with short- and long-term memory consolidation and learning.
• Sleep promotes cell regeneration. When the pituitary gland releases human growth hormones, your body begins regenerating cells and repairing muscle and tissue. Deep sleep also allows for more cerebrospinal fluid (CSF) to flow into your body, which clears out cell waste. CSF also washes away beta-amyloid—a toxic protein linked to Alzheimer’s disease—from neurons in your brain.
• Sleep supports your immune system. The processes that occur during REM sleep and non-REM sleep enable physical growth and a bolstered immune system. Sleep helps your body fight infection and illness, reduce inflammation, and boost the efficacy of vaccines.

Failing to get regular sleep or experiencing sleep deprivation can impair your mental and physical health. A persistent lack of sleep raises the risk of illness. Consistent sleep loss or operating on too little sleep may not show its effects at first, but it can become a source of cognitive impairment and poor overall health.

Neuroscientists have determined that the human body goes through four stages of sleep.

1. 1. NREM stage N1: This first stage of sleep contains the transition from wakefulness to sleep. Common behavior during this stage includes drowsiness, muscle relaxation, mild twitching, and slow eye movements. Alcohol use can trigger this stage of sleep, but it does not facilitate the more restful forms of deep sleep.
2. 2. NREM stage N2: Better known as light sleep, this sleep stage occurs the most frequently during a full night of sleep. During this stage, your body temperature drops, eye movements stop, and brain waves slow. 3.__ 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 in the brain, 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.
3. 3. 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. Characterized by rapid eye movement, this stage features intense brain activity. During this sleep stage, your breathing and heartbeat increase, but your muscles undergo paralysis to prevent you from physically acting out the events of your dreams. Your heart rate decreases as you cycle back into non-REM sleep.

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