Understanding sleep stages

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There are four stages of sleep: Non-REM (NREM) sleep (Stages 1, 2 & 3) and REM sleep. Periods of wakefulness occur before and intermittently throughout the various sleep stages or as one shifts sleeping position.

Wake is the period when brain wave activity is at its highest and muscle tone is active.

Stage N1 is the lightest stage of NREM sleep.  Often defined by the presence of slow eye movements, this drowsy sleep stage can be easily disrupted causing awakenings or arousals. Muscle tone throughout the body relaxes and brain wave activity begins to slow from that of a wake. Occasionally people may experience hypnic jerks or abrupt muscle spasms and may even experience a sensation of falling while drifting in and out of Stage N1.

Stage N2 is the first actual stage of defined NREM sleep.  Awakenings or arousals do not occur as easily as in Stage N1 sleep and the slow-moving eye rolls discontinue. Brain waves continue to slow with specific bursts of rapid activity known as sleep spindles intermixed with sleep structures known as K complexes. Both sleep spindles and K complexes are thought to serve as protection for the brain from awakening from sleep. Body temperature begins to decrease and heart rate begins to slow.

Stage N3 is known as deep NREM sleep. The most restorative stage of sleep, stage N3 consists of delta waves or slow waves.  Awakenings or arousals are rare and often it is difficult to awaken someone in Stage N3 sleep. Parasomnias (sleepwalking, sleep talking, or somniloquy and night terrors) occur during the deepest stage of sleep.

REM sleep, also known as rapid eye movement, is most commonly known as the dreaming stage.  Eye movements are rapid, moving from side to side and brain waves are more active than in Stages N2 & N3 of sleep. Awakenings and arousals can occur more easily in REM; being woken during a REM period can leave one feeling groggy or overly sleepy.

How Your Sleep Cycle Changes With Age

Sleep changes throughout a person’s life.  From a newborn, through toddler years, school age, adolescence and adulthood, sleep is changing.

Newborn (0 – approximately 4 months) Do not have distinctive sleep waves.  Sleep is categorized as “Active”, “Quiet” and “Indeterminate”. Active sleep is the equivalent to REM sleep and quiet sleep is equivalent to non-REM sleep.  A majority of the time, newborns are inactive sleep which allows for frequent arousals or awakenings; this is necessary for regular periods of feeding.

Infants (Approximately 4 months – 1 year) Standard sleep stage distinction is now apparent.  Sleep becomes more consolidated and sleeping routines can be developed, sleep is typically 10-13 hours per 24 hour period with 2-3 daytime naps occurring.

Toddlers (1 year – 3 years) With sleeping patterns fully developed, children spend approximately 25% in Stage 3 deep sleep with almost an equal amount of time in REM.  the average sleep time is 9.5-10.5 hours per 24 hour period. Typically naps will reduce to 1 per day most likely occurring early in the afternoon to allow for proper nighttime sleep.

Pre-School (3 – 6 years) Sleep time is similar to that of toddlers, approximately 9-10 hours per 24 hour period.  The afternoon nap usually subsides around 3-4 years for a majority of children. Stage 3 sleep still remains high in relation to total sleep time.

School Age (6 years – 12 years) Sleep time remains unchanged; 9-10 hours per 24 hour period and Stage 3 remains approximately 20-25% of total sleep time.  Restorative sleep is important for growth and development.

Adolescent (12 years and beyond) Sleep time for adolescents is approximately 9-9.5 hours per 24 hour period.  There are physiological changes in circadian rhythm that occur causing sleep onset to be later. This internal shift is the cause for many adolescents to have later lights out and the desire to want to “sleep in” in the morning.   As a person ages, the circadian rhythm shifts back, and sleep again appears to regulate to approximately 6.5-8 hours of sleep per 24 hour period as an adult.


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