Sleep has been identified as an essential human need; this is partly because of the metabolic activities that occur while the individual is sleeping. Normal sleep is divided into non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. NREM sleep comprises 75% to 80% of total sleep time (TST), and is characterized by relatively quiescent brain activity and decreased metabolic rate (Carskadon & Dement, 2000). NREM sleep consists of four stages (S1-S4), with each stage leading to a progressively deeper sleep. REM sleep follows slow wave sleep (SWS), or deep sleep, and increases over the night, comprising 20% to 25% of TST. REM sleep is characterized by an activated EEG pattern, muscle atonia, and episodic bursts of rapid eye movements. Normal sleep provides a period of physiologic and mental rest. During sleep, sympathetic tone decreases and parasympathetic tone increases, leading to a reduction in heart rate, arterial blood pressure, and cardiac output (Rosenthal, 1998). Deep sleep is theorized to be necessary for physiologic restoration. REM sleep is associated with dreaming, and is essential for maintaining emotional and cognitive well-being (Redline et al., 2004). Waking and consciousness depend on the activity, of neurons in the ascending reticular activating system of the brainstem. These neurons project into the thalamus, hypothalamus and basal forebrain and eventually send projections to the cortex. There are particular neurotransmitters, such as the catecholamines, acetylcholine, histamine, glutamate and aspartate, that are localized within the reticular formation and have important roles in cortical activation and arousal (Jones, 1989). Sleep-promoting neurotransmitters include gamma aminobutyric acid (GABA), adenosine, and melatonin. Specific stages of sleep are regulated by the turning "on" and "off" of various neurons. REM "on" cells use GABA, acetylcholine, and glutamine, whereas REM "off" cells use norepinephrine and serotonin." REM On cells" are cholinergic cells in the lateral pontine and medial medullary reticular areas that innervate the thalamus, hippocampus and hypothalamus. These cells discharge at high rates during REM and show little or no activity during NREM."REM Off cells" are noradrenergic and serotonergic cells found in the locus coeruleus and raphe. These are cells which are slow or silent during REM sleep. Affecting levels of norepinephirne or serotonin can have an effect on REM sleep (Hoyt, 2005). Sleep regulation is a balance between a homeostatic sleep need and an intrinsic body clock, or circadian pacemaker. Located in the suprachiasmic nucleus, the circadian pacemaker determines the onset and termination of sleep, and is partially regulated by environmental www.intechopen.com
Progress in Hemodialysis -From Emergent Biotechnology to Clinical Practice