The two main types of sleep are rapid-eye-movement (REM) sleep and non-rapid-eye-movement (NREM) sleep.
NREM is broken into three segments: Stage 1 (known as N1), Stage 2 (N2), and Stage 3 (N3). REM also has segments, although they are rarely written about and not useful to talk about for our purposes (and I’m sure you’re just dying to know).
A breakdown of the phases:
- Stage 1 — Drowsy, relaxed state between being awake and nodding off. Your breathing slows, and blood pressure decreases. About ~5% of total sleep time.
- Stage 2 — Light sleep; your body temperature cools and heart rate drops. You become disengaged with your surroundings, although you can generally still easily be woken up. About ~45% of sleep time.
- Stage 3 — Deep sleep; it’s very hard to wake up from this state. A number of restorative processes happen in the body that help healing, memory and more. About ~25% of sleep time.
- REM — Heart and respiration rates increase and become irregular. Your body paralyzes itself to inhibit movement. Your body temperature further decreases. Most of your dreams happen in this state. Interestingly, unlike Stages 1–3, brain activity in REM looks very similar to being awake. About ~25% of sleep time.
A typical night’s sleep goes like this: we enter into N1, which initially lasts a few minutes, then quickly move to N2, then N3, back to N2, and then REM. This whole cycle takes about 90 minutes. As the night progresses, we spend less time in N3, and predominately keep switching between N2 and REM (and REM duration gets longer and longer).