Sleep Stages Explained: Light, Deep and REM Sleep
Sleep is not a single state. Each night you move through several stages in repeating cycles, and knowing them explains why some mornings feel much easier than others.
By the Sleep Calculator team at Digimarket Group SIA · Updated 4 November 2026
The stages
| Stage | What happens | Share of night (adults, approx.) |
|---|---|---|
| N1 (light) | Transition from wakefulness; easy to wake; muscle twitches common. | ~5% |
| N2 (light) | Heart rate and temperature drop; sleep spindles appear on EEG; linked to memory consolidation. | ~45–55% |
| N3 (deep / slow-wave) | Slow brain waves; hardest to wake from; important for physical recovery and growth hormone release. | ~15–25% |
| REM | Rapid eye movements, vivid dreams, brain activity close to waking, most muscles temporarily relaxed. | ~20–25% |
How cycles change through the night
A cycle runs N1 → N2 → N3 → N2 → REM, then starts again. Early in the night, cycles contain long periods of deep N3 sleep and short REM periods. In the second half of the night deep sleep shrinks and REM periods lengthen. That is why cutting sleep short in the morning mainly costs REM sleep, while going to bed very late can reduce deep sleep.
Sleep inertia
Waking suddenly from deep N3 sleep often causes “sleep inertia”: grogginess, confusion and slower reactions that can last from a few minutes to half an hour. Waking from lighter N2 or at the end of REM usually feels easier. Sleep calculators try to place your alarm near the end of a cycle for this reason.
Changes with age
Infants spend about half of their sleep in REM. Deep sleep is greatest in childhood and declines through adulthood; older adults often have lighter, more fragmented sleep and wake more often in the night.
What disrupts the stages
- Alcohol may help you fall asleep but suppresses REM early and fragments sleep later.
- Caffeine can reduce deep sleep even when taken six hours before bed.
- Irregular schedules shift your body clock and reduce sleep quality.
- Sleep apnea repeatedly interrupts sleep, often without the person noticing.