Sleep cycle

Sleep isn't static, but a cycle (dynamic).

Many people think of sleep as a kind of "off switch," but in reality, our brain goes through an ingenious pattern of different sleep stages every night. These are roughly divided into two main types: non-REM sleep and REM sleep. Together, they form a sleep cycle that repeats about four to six times per night.

What is non-REM sleep?

Non-REM sleep (Non-Rapid Eye Movement) is the "deep foundation" of our sleep. It is divided into three stages, which range from light to deep sleep:
1. Stage N1: This is the transition from wakefulness to sleep. It lasts only a few minutes. Brain waves slow slightly, and you can still easily be awakened.
2. Stage N2: This is light but more stable sleep. Your heart rate and body temperature drop. The brain exhibits so-called sleep spindles and K-complexes: short electrical bursts that help the brain suppress external stimuli.
3. Stage N3: Also called deep sleep or slow-wave sleep. Slow delta waves dominate in the EEG (brain scan). This stage is essential for physical recovery, tissue growth, immune function, and the removal of waste products from the brain.

During non-REM sleep, your body is at rest: heart rate, breathing, and blood pressure are low and stable. Muscles remain partially active, so you are not completely paralyzed.

What is REM sleep?

REM sleep (Rapid Eye Movement) is a paradoxical (contradictory) sleep stage: your muscles are completely relaxed and almost paralyzed (atonia), but your brain is almost as active as when you are awake.

Characteristics:
• Rapid eye movements (hence the name).
• Vivid, often bizarre dreams. During REM sleep, information is processed and emotionally charged memories are integrated. • REM sleep plays a crucial role in learning, memory, and emotional regulation.
• Brain activity resembles that of a waking state, but motor signals are blocked, preventing you from acting out your dreams.

The ratio of non-REM to REM sleep

An average sleep cycle lasts about 90 to 120 minutes. You always begin with non-REM sleep, which gradually becomes deeper. This is followed by a shorter REM phase. As the night progresses, the ratio shifts: the first half of the night consists mainly of deep non-REM sleep, while later REM sleep actually increases.

Why are both necessary?

• Non-REM sleep is the body's restorative phase: muscle growth, cell repair, and immune function occur here. A lack of deep non-REM sleep leads to physical exhaustion.
• REM sleep supports mental processes such as memory consolidation, problem-solving, and emotional processing. Too little REM sleep can contribute to irritability, difficulty concentrating, and mood disorders.

In summary:

Non-REM and REM are like two dance partners in a nighttime choreography. They alternate in a fixed rhythm, each fulfilling its own essential function. Disruption of this subtle balance—for example, due to stress, sleep apnea, alcohol, or sleep debt—can have a major impact on both your physical and mental health.

What happens in each stage?

✅ Stage N1
• Transition from wakefulness to sleep
• Slight drowsiness, eyes moving slowly.

✅ Stage N2
• Deeper, lighter sleep
• Heart rate and temperature drop
• Sleep spindles protect sleep from stimuli.

✅ Stage N3 (deep sleep)
• Recovery of the body and immune system
• Slow brain activity: delta waves
• Difficult to wake up.

✅ REM sleep
• Dreaming!
• Muscles inactive, brain active
• Rapid eye movements
• Processing emotions and memories.

An average night consists of 4-6 cycles. The first half of the night contains more deep sleep (N3). The second half of the night contains longer REM phases, lighter sleep.

Practical example of a night:

1️⃣ 9:30 PM – 11:00 PM ➜ Non-REM sleep dominates (lots of N3 sleep)
2️⃣ 11:00 PM – 12:30 AM ➜ Short stretch of REM sleep
3️⃣ 12:30 AM – 2:00 AM ➜ Again, lots of Non-REM sleep, a little REM sleep
4️⃣ 2:00 AM – 4:00 AM ➜ REM phases become longer
5️⃣ 4:00 AM – 6:00 AM ➜ Mainly REM sleep, dreaming!