Welcome to 2 minute neuroscience, where I
explain neuroscience topics in 2 minutes or less. In this installment I will discuss the
stages of sleep. Sleep stages are defined based on the measurement
of electrical activity in the brain using an electroencephalogram, or EEG. An EEG represents
fluctuations in brain electrical activity in voltage as a waveform of variable frequency
and amplitude. The American Academy of Sleep Medicine classifies
sleep as consisting of 4 stages. The first two stages involve drowsiness and light sleep.
When someone begins to fall asleep, they enter stage 1, during which an EEG records low-amplitude
waves of mixed—but mostly high—frequencies. Next, the person enters stage 2 sleep. This
is characterized by the presence of phenomena on an EEG known as sleep spindles and K complexes.
Sleep spindles are trains of high-frequency waves. A K complex involves a biphasic wave
that stands out from the rest of the EEG. Stage 3 sleep is also known as slow-wave sleep
or deep sleep. In stage 3, delta waves, which are low-frequency, high amplitude waves, make
up at least 20% of brain activity. Stage 3 sleep is thought to be especially important
to overall restfulness. Next, the sleeper passes rapidly back through stage 2 and stage
1 before entering rapid eye movement, or REM, sleep. In REM sleep, EEG activity resembles
what’s seen in stage 1 or restful waking. During REM sleep, the muscles are completely
relaxed and limp but the eyes are moving rapidly. This is the time of sleep when our most vivid
dreams are likely to occur. After REM sleep, the person will sometimes
awaken briefly but then will move through the sleep stages again, in order. Most people
will repeat this cycle 4-5 times a night, with each cycle lasting about 90-110 minutes.