Sleep Stages Explained

Each night your brain cycles through multiple sleep stages, each serving a distinct restorative function. Understanding these stages helps you interpret what sleep quality truly means.

The Four Sleep Stages

Light Sleep

Light sleep is the transitional phase and typically the largest portion of the night (40–60%). During light sleep:

• Muscles gradually relax; heart rate and breathing slow down
• The brain begins initial memory processing
• You're easily awakened by external sounds

Light sleep isn't "bad" sleep — it's a normal and necessary part of sleep architecture.

Deep Sleep

Deep sleep is the core stage for physical recovery. During deep sleep:

• Growth hormone is released, promoting tissue repair
• The immune system strengthens
• Metabolic waste is cleared (glymphatic system flush)
• You're hardest to wake up

Deep sleep concentrates in the first half of the night and naturally decreases with age.

REM Sleep

REM (Rapid Eye Movement) sleep is when the brain is highly active (typically 20–25%). During REM:

• The brain processes emotional memories and consolidates learning
• Eyes move rapidly; muscles are temporarily paralyzed
• Most vivid dreaming occurs in this stage

REM sleep concentrates in the second half of the night and early morning.

Awake Periods

Brief awakenings throughout the night are entirely normal. You typically won't remember them. Prolonged or frequent awakenings may reduce recovery efficiency.

Healthy Sleep Architecture

A typical healthy adult sleep structure looks roughly like:

• Light sleep: 45–55%
• Deep sleep: 15–25%
• REM sleep: 20–25%
• Awake: < 10%

Mellows analyzes your stage distribution and compares it against your personal baseline to help you understand whether your nightly architecture is in good shape.