NORMAL SLEEP IN CHILDREN
Sleep is the primary brain activity of the youngest children with an estimated 9500 hours (13 months) spent in sleep by the time a child is 2 years old (as compared to 8000 hours awake).1 The sheer proportion of time young individuals spend in this state suggests its importance in appropriate development. Many have similarly argued that sleep is of particular importance in developing animals.2-5 Although the functions of sleep are still not well elucidated, roles for sleep in cognition, learning, and memory consolidation are well founded.4,6-10 It therefore stands to reason that issues of sleep may have developmental consequences, both for typically developing children and those with atypical neurodevelopment.
Sleep is a physiologic, reversible, and recurrent behavioral state of perceptual disengagement from the surrounding environment and relative unresponsiveness to external stimuli.11 It is dynamic, resulting from the complex interplay of physiologic and behavioral processes that can be broadly segmented into (1) non–rapid eye movement (NREM) sleep, a sleep state characterized by unconscious or bland thoughts, a synchronized electroencephalogram (EEG) dominated by slower frequencies in the delta (0-4 Hz) and theta (4-7 Hz) ranges, and low sympathetic tone; and (2) rapid eye movement (REM) sleep, a dreaming or paradoxical sleep characterized by bursts of fast saccadic eye movements, a low voltage, mixed EEG pattern, variable sympathetic tone, and atonia of all voluntary muscles (except the extraocular muscles and the diaphragm).12
Age-Related Changes in Sleep Time, Sleep Architecture, and Sleep Staging
Sleep in most age groups (except infants) is categorized into 4 stages, primarily using EEG characteristics: stage N1, stage N2, stage N3 (deep sleep, delta wave sleep, or slow wave sleep), and stage REM. Stage N1 is typically characterized by slow, conjugate, sinusoidal eye movements of greater than 500 milliseconds. A transition from wakefulness to stage N1 often includes a slowing of background frequencies to low-amplitude, mixed-frequency (predominantly 4-7 Hz) EEG activity. Sharply contoured vertex waves of less than 0.5 seconds may also be present; these waveforms typically appear at 4 to 6 months postterm. Stage N2 is characterized by the presence of K complexes and/or sleep spindles on EEG. A K complex is a well-defined, negative, sharp deflection, usually maximal in amplitude in the frontal derivations, lasting for 0.5 seconds or more. Sleep spindles are trains of 11- to 16-Hz sinusoidal waves whose amplitudes are maximal in central derivation. K complexes are seen in the sleep EEGs of most infants by 3 to 6 months postterm. Sleep spindles may be seen by age 6 weeks to 3 months postterm and are present in all typically developing infants by 2 to 3 months of age.13 The physiologic significance of these 2 wave types is unclear, but intriguing functions have been proposed. K complexes have been hypothesized to be an index of sleep arousals14-16 or to protect ...