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Sleep stages, REM and the sleep cycle
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We spend about a third of our lives asleep. Sleep is not one state: the brain passes through different stages in a repeating cycle.
The four stages and REM
Brain activity in sleep can be measured with an EEG, which records brain waves.
Stage 1: light sleep, between waking and sleeping. Easily woken; you may feel like you are falling.
Stage 2: still fairly light. Breathing and heart rate slow, body temperature drops, and brain waves show short bursts called sleep spindles.
Stages 3 and 4: deep sleep (slow-wave sleep). Slow, large delta waves. Very hard to wake someone. Growth hormone is released.
REM (rapid eye movement) sleep: the eyes dart about, the brain is very active, almost like being awake, but the body's muscles are paralysed. Most vivid dreams happen in REM.
Stage 1: light sleep, between waking and sleeping. Easily woken; you may feel like you are falling.
Stage 2: still fairly light. Breathing and heart rate slow, body temperature drops, and brain waves show short bursts called sleep spindles.
Stages 3 and 4: deep sleep (slow-wave sleep). Slow, large delta waves. Very hard to wake someone. Growth hormone is released.
REM (rapid eye movement) sleep: the eyes dart about, the brain is very active, almost like being awake, but the body's muscles are paralysed. Most vivid dreams happen in REM.
The sleep cycle
The stages happen in a cycle lasting about 90 minutes: stage 1, 2, 3, 4, then back up through 3 and 2, then REM.
We have about 4 to 5 cycles a night.
Early in the night, cycles contain more deep sleep; later in the night, REM periods get longer.
Babies spend a much larger share of their sleep in REM than adults.
We have about 4 to 5 cycles a night.
Early in the night, cycles contain more deep sleep; later in the night, REM periods get longer.
Babies spend a much larger share of their sleep in REM than adults.
Functions and benefits of sleep
Restoration: deep sleep helps the body repair tissue and grow (growth hormone); REM may restore the brain.
Memory and learning: sleep, especially REM, helps consolidate (strengthen) memories of what we learned that day.
Energy conservation: sleeping saves energy at times when it would be hard or dangerous to be active.
Benefits: better concentration, mood and immune function. Lack of sleep causes tiredness, poor attention and irritability.
Memory and learning: sleep, especially REM, helps consolidate (strengthen) memories of what we learned that day.
Energy conservation: sleeping saves energy at times when it would be hard or dangerous to be active.
Benefits: better concentration, mood and immune function. Lack of sleep causes tiredness, poor attention and irritability.
A person sleeps for 7.5 hours. If each sleep cycle lasts about 90 minutes, about how many cycles do they have?
- 7.5 hours = 7.5 × 60 = 450 minutes.
- 450 ÷ 90 = 5
Answer: About 5 cycles.
Sleep has four stages plus REM. Stages 3 and 4 are deep, slow-wave sleep; REM has an active brain, paralysed body and vivid dreams. A cycle lasts about 90 minutes and repeats 4 to 5 times a night, with more REM later. Sleep helps restore the body, consolidate memories and conserve energy.
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