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[by Tamara Sellman RPSGT CCSH for Advanced Cardiovascular Sleep Disorders Center]

Image courtesy University of Minnesota Libraries (open.lib.umn.edu )

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Sleep architecture describes the parts of the biological process of sleep that create a whole "picture" of your night while asleep.

Measuring the quality of one's sleep architecture includes data interpretations of components such as sleep stages, cycles, and sleep-wake phasing. 

THE 4 SLEEP STAGES

STAGE 1 (N1)

[Up to 30 minutes, or 5 percent of total sleep time]

This is that light sleep you encounter as you transition between sleep stages and from wakefulness to sleep. You spend very little time in N1 (stage 1) sleep, usually just at the very beginning of the sleep period following wakefulness, in quick bursts between other stages of sleep, and in that last period of sleep just before waking up in the morning.
 

STAGE 2 (N2)

[4 hours, or 50 percent of total sleep time]

You spend half of your night in this stage of sleep. This is considered light sleep, a time in which several things take place in the body to prepare you for other stages of sleep. Brainwave activity, breathing rate, and pulse slow down and your core body temperature falls.

STAGE 3 (N3)

[1 to 1.5 hours, or 15 to 20 percent of total sleep time]

Now you’re into the deepest stage of sleep. This is also referred to as “slow wave sleep” or “delta sleep,” the latter term describing the large delta wave patterns that are characteristic of deep sleep. This is the time in your night when you are least likely to awaken and can be hard to rouse. It is also the time when the brain releases human growth hormone (HGH) into the bloodstream to do repair work related to injuries and illness and to help regenerate your cells.

If you’re looking for a Stage 4 definition, there isn’t one. That stage of sleep was merged with Stage 3 several years ago because it only reflects an even deeper layer of sleep, but is otherwise indistinguishable from Stage 3.

STAGE REM (R)

[1.5 to 2 hours, or 20 to 25 percent of total sleep time]

Also known as “dream sleep” or “active sleep,” the REM stage is identified by noticeable rapid-eye movements that occur during this stage. It is thought to be the key time when the brain deep cleans itself of waste products, processes learned information, and consolidates memory. The brain becomes as active (or maybe even more active) during this stage than when it is awake.

A lot of changes in the brain and body take place at this time: heart rate, breathing, and brain activity increase. Meanwhile, a physiological “gate” in the brainstem cuts off nerve impulses to the body (below the chin level), basically creating a temporary paralysis. Only your diaphragm, the muscle below your rib cage that facilitates breathing, continues to work during REM.

REM takes place about 90 minutes (1.5 hours) into the first sleep cycle (see below). 

WHAT A SLEEP CYCLE LOOKS LIKE

We experience these stages of sleep as a sequence of sleep cycles in which the stages are repeated. Sleep professionals identify these cycles and stages in a chart known as a histogram (see above). This marks the descent from wake to sleep, then from cycle to cycle, until the sleep period is complete.

A complete sleep cycle starts with N1 sleep, slips into N2 sleep, then N3 sleep, then REM sleep. Once REM is completed, the cycle repeats itself. Those who sleep a full 8 hours a night enjoy up to 5 full cycles of sleep a night.

It’s worth noting that the way you experience these sleep stages changes over the course of the night. 

From bedtime through the first half of the night, you are most likely to experience long periods of both light sleep and deep sleep. You will still have 1 to 2 REM periods, but they will be very brief (10 minutes).

Into the second half of the sleep period, sleep cycles change significantly to feature far more REM sleep (each stage could last an hour) and minimal N3 sleep. You will still have a little bit of N2 sleep and brief transitions of N1 sleep as you slide between N2 and REM sleep, but mostly you will be in dream mode through as many as 3 cycles until you awaken in the morning.

WHY MEASURING SLEEP ARCHITECTURE MATTERS

What’s described here defines a healthy sleep pattern. If you have sleep problems and undergo a sleep study, it's very likely your personal sleep architecture will differ from these patterns. People with sleep disorders suffer the loss of certain kinds of sleep, most notably N3 (deep) sleep and REM (dream) sleep. Since both stages are key to optimal health, it’s the loss of these stages that is the most concerning.

THE PROBLEM OF SLEEP FRAGMENTATION

The following sleep problems can greatly reduce your chances of achieving N3 sleep, REM sleep, or both. 

• Sleep apnea (all forms)
• Snoring
• Periodic limb movement disorder
• Nocturia (frequent need to urinate at night)
• Pain from injuries, illness, chronic diseases and disorders
• Side effects of medications
• Gastroesophageal reflux disease
• Poor sleep hygiene (late caffeine consumption, alcohol at bedtime, poor sleeping environment)

Note: this is not an exhaustive list! There are many more explanations for disrupted sleep than are included here.

The concerns listed above are problematic because the brain will be interrupted from all stages of sleep due to physical changes in the body that cause the release of stress hormones. These changes fragment one's sleep architecture, eliminating opportunities for sleep cycles to process to their completion.

These changes can include pauses or disruptions in breathing, heartburn leading to reflux, involuntary leg movements that briefly and repeatedly arouse the brain, shifts in blood glucose, urgent bladder signals, and other neurochemical changes caused by chronic health concerns (i.e. epilepsy, diabetes, multiple sclerosis) as well as substance withdrawal, or the side effects of certain medications.

To see it another way, if you’re in stage 2 sleep, you’ll shift to stage 1 sleep or even wake up due to these changes. When there are too many stress hormones in the system, deep and dream sleep cannot be achieved for very long, if at all.

THE PROBLEM OF SLEEPLESSNESS

Another problem concerning incomplete sleep cycles involves the nature of sleeplessness (insomnia). If you struggle to fall asleep at night, this can mean your sleep period can be shortened by at least 1, if not 2 or even 3, full sleep cycles. That’s a lot of REM sleep lost at the end of the night which you need to maintain cognitive, emotional, psychological health.

Insomnia can be caused by many things both internal and external and should be something you only experience occasionally. It becomes chronic and a danger to your health if it occurs most nights for a period of at least 3 months. If you routinely do not get adequate sleep (up to at least 7 hours) in spite of opportunities to do so, you need to raise this concern with your doctor. Insomnia is not something you have to live with; there are treatments (both pharmaceutical and non-drug) that can help you achieve adequate sleep. It might also require you to address "second-hand sleep problems" caused by bad personal habits or your environment.

ABOUT SLEEP-WAKE PHASING

Our circadian rhythms coax us into sleep at night and wakefulness during the day following patterns that match the light-dark cycles of the Earth. We gradually "phase" into sleep when the sun goes down, then "phase" into wakefulness at dawn.

Disruptions to the circadian system, which enables us to stay awake for two thirds of the day and to sleep for the final third “phase,” can also lead to shortchanges in these critical sleep cycles. Circadian disruptions can include shift work disorder caused by working evenings or overnight, jet lag (even if only temporarily), and circadian sleep disorders which delay the sleep phase (you’re a night owl) or have inconsistent or random sleep phases that do not match normal light-dark cycles (such as non-24 sleep-wake disorder).

In the case of those who like to keep later hours, only those who are allowed to sleep a full 8 hours can enjoy all their sleep cycles. Unfortunately, many who prefer the night light (or work at night) don’t always get the chance to sleep in due to the demands of work, school, or family. This is also extremely problematic for those who have "free running" circadian rhythms which can be incredibly disruptive to day-to-day living. 

ABOUT REM REBOUND

You might be surprised to learn that for some patients undergoing sleep studies, noninvasive therapies to treat certain apparent problems (such as CPAP for sleep apnea) often means they will experience a phenomenon known as "REM rebound," in which they experience long periods of uninterrupted REM. The brain, when given the opportunity to consolidate sleep cycles after long-term sleep deprivation (known collectively as sleep debt), will take advantage of the opportunity to do its important work during this cycle when it has been denied the chance previously.

Sleep architecture is the foundation for optimum health

When you hear it repeated that you need (as an adult) at least 7 hours of sleep, this is the reason why. If you shortchange yourself deep and dream sleep, that’s the time when systemic inflammation can set in.

Systemic inflammation describes a continuous body-wide state of stress which can upset the balance of hormone levels, which creates problems for the brain, the body’s many organs, and even processes that take place at the cellular level. Being in a state of systemic inflammation leaves you wide open to the ravages of infection, disease, and physical and mental dysfunction.

Sleep disorders generally lead to systemic inflammation, which then lead to these common outcomes, if left untreated: high blood pressure, high stroke risk, heart disease, insulin resistance and diabetes, mood disorders, behavioral problems, and an increase in accidents (motor vehicle or workplace) and mistakes due to sleep deprivation.

Any sleep period that measures less than 7 hours (on a regular basis) means you are being robbed of life-sustaining biological processes that are put in place to keep you healthy. Once in a while, a few hours of lost sleep isn't going to do much harm. It's when it happens night after night that you need to be concerned.

Making sleep a priority and treating your sleep problems (however big or small) are two ways you can maintain good health and well being.