Sleep and Sleep Disorders Part 2 - Understanding Sleep

This is the second part of Sleep and Sleep Disorders

Understanding Sleep

Sleep is considered now to be an  active state of being; the mind is a 24-hour mind, and it does stop  working, a surprising fact of which we often are not aware. We spend roughly one-third of our lives sleeping in this active state. Although the specific function of sleep still eludes researchers, many substantiated theories point to the necessity of good-quality  sleep to our existence and overall well-being.  Different brain cell chemicals and neurotransmitters regulate  our sleep-wake states by acting on a variety of cells in the brain. Sleep-wake functioning is analogous to a light switch with on and off features. A type of switch setting systematically turns on sleep,  including turning on different types of sleep, and systematically turns off each sleep stage correspondingly. This switch setting easily explains how our sleep progresses from light sleep to deep sleep and then transcends to dream, or REM, sleep. The neurochemical signaling begins when we fall asleep. An exact sleep chemical is still considered to be controversial, yet a number of studies suggest adenosine, which is a nucleoside, binds to cells and causes a cascade of events that promote drowsiness.

Adenosine levels in the blood increase during waking hours and have a cumulative effect throughout the day, causing drowsiness.  During sleep, adenosine levels decline, promoting wakefulness toward the end of the nocturnal period. The cycle begins again once we are awake. It is not surprising that stimulants such as caffeine cause wakefulness because caffeine competes for the same receptors to which adenosine binds. If more receptors have caffeine bound to them, sleepiness is less likely to happen.

 

Sleep Stages

Sleep architecture refers to the various stages in the sleep-wake cycle, typically defined by a brainwave (EEG) recording. In healthy individuals without sleep problems, these stages occur in a regular pattern throughout a 24-hour period. Sleep is of two types, dream or rapid eye movement (REM) sleep that occurs every 1.5 hours throughout the sleep interval, or 18 percent to 25 percent of the  sleep period. 7,8 REM periods vary from a number of minutes to an  hour or more. REM sleep has a characteristic physiological pattern distinguished by the lateral saccadic (left to right) rhythm of the  eyes, absence of muscle movement (atonia), and heightened cardiovascular arousal.

Non-REM (NREM) sleep occupies a greater portion of the sleep period. NREM is further subdivided into stages 1, 2, 3, and 4, with corresponding physiological activity to each.

Stage 1 is considered light sleep and is estimated to be approximately 5 percent of the sleep period.

Stage 2 sleep is about 60 percent of the sleep interval and is considered formally to be sleep.

Stages 3 and 4 sleep are often collapsed together and are classified as deep sleep, a physiological event characterized by slow brain wave patterns and increased immune system activity. Non-REM makes up approximately 10 percent to 15 percent of the sleep period. A night of sleep is characterized as a predicted pattern beginning with the initiation of sleep onset (Stage 1) and progression to Stages 2, 3, and 4. Within 90 minutes after sleep onset, the first REM episode (generally four to five REM episodes per night) occurs. Following this sleep period, the cycle repeats itself, with at least four cycles of sleep per night. An excess or deficit in the amount of a type of sleep (e.g.,no REM), a misordering of the timing of sleep (e.g., sleep begins with REM), or an intrusion into sleep represent conditions for further study to determine if a sleep disorder exists.

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