Your sleep at night is not one long continuous block — it cycles through distinct stages throughout the night.
Non-REM sleep: Science describes three NREM stages, but Apple Health and most consumer devices combine them into two categories:
• Core Sleep: Corresponds to NREM Stage 1 + Stage 2 combined. This is the largest portion of the night, typically 40–55% of total sleep. Stage 1 is a brief transitional phase at sleep onset (a few minutes, easily awakened); Stage 2 is true light sleep, where heart rate slows, body temperature drops, and brain activity decreases. Together they appear as "Core Sleep" in Apple Health and DotSleep.
• Deep Sleep: Corresponds to NREM Stage 3, also called slow-wave sleep — the hardest stage to wake from. Growth hormone is released, cells are repaired, and immune function is strengthened. Deep sleep is concentrated in the first half of the night and typically makes up 10–20% of total sleep in adults.
REM sleep: The eyes move rapidly under closed lids, and brain activity is close to waking levels. This is the primary dreaming stage, critical for emotional regulation and memory consolidation. REM sleep is concentrated in the second half of the night.
Sleep cycles: Each cycle lasts roughly 80–100 minutes, with 4–6 cycles per night. As the night progresses, deep sleep decreases and REM sleep increases — which is why you often remember dreams upon waking in the morning.