SLEEP

A. Define: a state of reduced responsiveness to and interaction with the environment; can be readily reversed (arousal)

B. Function: “restoration” of brain and body function; exact functions are unclear but include

1. anabolic metabolic activity (e.g. growth hormone)
2. optimum immune system function
3. brain development in infants and young children
4. memory consolidation

Note: prolonged sleep deprivation leads to mental and emotional and physical disturbances, among which are

reduced immune system function
reduced rate of wound healing
reduced cognitive function (learning, memory)
attention deficit
hallucinations
tremor
increased irritability
yawning

Note: most animals and all "higher" animals sleep

C. Sleep Classification

Note: sleep stages are defined by the EEG during each stage and whether the eyes are moving (even though the eyes are closed)

Note: eye movements during sleep are measured by the electrooculogram (EOG) using electrodes placed adjacent to the eye or by special contact lenses containing pickup devices

1. REM (Rapid Eye Movement) Sleep

a. EEG similar to the awake state (“paradoxical sleep”)
b. body mainly motionless except for eye movements
c. dreaming; dreams can be vivid and complex
d. increased metabolic rate (compared to non-REM sleep)
e. sympathetic nervous system active (increase heart and respiration rate)

2. Non-REM Sleep

a. slower, larger amplitude EEG waves
b. occasional involuntary movements (sometimes talking, walking, screaming)
c. no (or little) dreaming; probably little mental activity
d. decreased metabolic rate
e. parasympathetic predominance (heart rate, respiration rate decreased)

D. Sleep Cycle: denoted by “stages”

1. REM Sleep

2. Non-REM Sleep Stages (progressively deeper sleep)

a. Stage 1: transitional sleep, alpha rhythm, few minutes duration, most easily awakened

b. Stage 2: sleep spindles, lasts 5-15 minutes

c. Stage 3: large amplitude delta rhythm; no body or eye movements

d. Stage 4: deepest stage of sleep, large amplitude low frequency waves (slow wave sleep), may last 20-40 minutes, most difficulty in arousal

Note: when sleepwalking occurs, it is during stage 4 sleep (about 40% of children sleepwalk). Although the eyes are open and the person can avoid obstacles and climb stairs during sleep walking, cognitive function and judgment are impaired. Bed wetting (nocturnal enuresis) also occurs during slow wave sleep. However, bruxing occurs during REM sleep and light sleep (Stages 1 & 2).

3. Progression from Awake, to REM sleep, then through progressively deeper non-REM stages; then reverses through the stages to REM sleep; several of these cycles occur throughout the night

As the night progresses, a reduction in duration of non-REM sleep (particularly stages 3 and 4) and an increase in REM sleep. The longest REM may last 30 to 50 min., followed by a refractory period of 30 min. of non-REM (i.e., before another REM can begin)

D. Changes with Age

F. Pathophysiology

1. Insomnia: inability to obtain sufficient sleep to maintain normal function despite adequate opportunity for sleep; various causes

2. Narcolepsy: irresistible urge to sleep during daytime activities; sudden onset of REM sleep

3. Coma - person cannot be aroused by strong sensory stimuli; a serious condition that can be produced by brain damage, drug overdose, hypothermia, and certain toxic-metabolic disorders. A comatose person does not exhibit REM activity, circadian rhythms or the periodic changes that characterize normal sleep, and has a general lowering of cerebral metabolism

G. Neurophysiological Basis of Sleep

1. Neurons containing norepinephrine (locus coeruleus of the pons) and serotonin (raphe neurons, midbrain) discharge during waking and enhance the waking state.

2. Cholinergic neurons from pontine reticular formation are active during REM sleep

3. Thalamus activity during deep sleep apparently blocks the flow of sensory input to the cortex

4. Descending pathways inhibit motor neurons particularly during REM sleep

F.  Sleep Promotion

1.  Reduced external stimulation

2.  Circadian rhythm, mediated by the hypothalamus and increased pineal gland secretion of melatonin

3.  Sleep drive (lack of sleep)