sleep stages
sleep is not a uniform state, but a dynamic process made up of distinct stages that repeat in cycles throughout the night. each of these stages has specific characteristics and serves a function in the body’s balance. understanding how they are structured helps explain why sleep paralysis can occur.
a cyclical process
section titled “a cyclical process”during the night, the brain does not remain in a single state. sleep is organised in cycles lasting approximately 90 to 110 minutes. each cycle includes a sequence of stages that repeat several times, alternating between deep sleep and states closer to wakefulness.
non-rem sleep stages (nrem)
section titled “non-rem sleep stages (nrem)”the first stages of sleep belong to non-rem sleep, which is divided into three progressive phases.
stage n1: transition to sleep
section titled “stage n1: transition to sleep”this is the lightest stage and marks the shift from wakefulness to sleep. at this stage, sleep is unstable and it is easy to wake up.
it is characterised by:
- decreased muscle activity
- reduced awareness of the environment
- brief or fragmented sensations
stage n2: light sleep
section titled “stage n2: light sleep”this stage represents a significant portion of total sleep time (between 40 and 50% in adults). although the person is asleep, they can still wake up with relative ease.
during this stage:
- brain activity slows down
- responsiveness to external stimuli decreases
- sleep spindles and k-complexes appear — electrical patterns that consolidate memory and block sensory processing
- the body continues to relax
stage n3: deep sleep
section titled “stage n3: deep sleep”also known as slow-wave sleep. this is the deepest stage of sleep and is associated with physical recovery. at this stage, awareness of the environment is practically nonexistent.
it is characterised by:
- slow, synchronised brain activity
- difficulty waking up
- restoration of the body
rem sleep: mental activity and atonia
section titled “rem sleep: mental activity and atonia”after the nrem stages, the brain enters rem sleep. this stage has markedly different characteristics: brain activity approaches that of wakefulness, the most vivid and structured dreams occur, and the eyes make rapid movements beneath the eyelids.
- increased brain activity
- vivid and structured dreaming
- rapid eye movements
- inhibition of voluntary muscles (muscle atonia)
muscle atonia during rem sleep prevents the body from acting out the movements that occur in dreams — an essential protective function that, when it becomes desynchronised, underlies sleep paralysis.
the relationship with sleep paralysis
section titled “the relationship with sleep paralysis”sleep paralysis is directly linked to rem sleep. under normal conditions, upon waking, muscle atonia disappears and consciousness activates simultaneously. however, when this process becomes desynchronised, consciousness returns before the body has exited rem atonia.
the key to the phenomenon lies not in the stages themselves, but in how the transitions between them occur. when the shift between sleep and wakefulness is not completed in a synchronised way, intermediate states can emerge in which elements of both coexist: the mind is awake, but the body remains immobilised by muscle atonia. that gap is sleep paralysis.
cycles throughout the night
section titled “cycles throughout the night”sleep cycles are not identical throughout the night. in the early hours, deep sleep (n3) predominates, while later in the night the duration of rem sleep increases. this explains why sleep paralysis is more likely to occur upon waking in the morning, after sleep interruptions, or during fragmented sleep: these are the moments when rem sleep is longest and most intense, and therefore the risk of a desynchronised transition is greatest.