Sleep in Infants and Young Children: Part One: Normal Sleep

Katherine Finn Davis RN, MSN, CPNP; Kathy P. Parker PhD, RN, FAAN; Gary L. Montgomery MD

Disclosures

J Pediatr Health Care. 2004;18(2) 

In This Article

Regulation of Sleep and Sleep Stages

Two main processes are believed to regulate sleep and wakefulness: (1) the circadian process, an internal rhythm or clock that dictates periods of activity (wakefulness) and inactivity (sleep) based on a light-dark cycle, and (2) the homeostatic process, in which the requirement for sleep builds during waking hours and is relieved by sleep (Davis, Heller, & Frank, 1999). Although the circadian and homeostatic processes are distinct and function independently, together they influence the timing and duration of sleep and wakefulness. During sleep, an ultradian rhythm determines the timing and duration of sleep states, each of which are quite different (Zee & Turek, 1999).

The circadian rhythm (derived from the Latin term circa diem, translated "about a day") functions as an internal clock, which incorporates cues from the external environment to regulate the timing of sleep and wakefulness. The circadian rhythm is actually about 25 hours in absence of synchronization with cues from the environment, a bit longer than our 24-hour clock day. Light exposure signals waking and darkness signals sleep, thus synchronizing the internal circadian rhythm to the external environment (Zee & Turek, 1999). Therefore, not surprisingly, changes in exposure to light or darkness can shift the circadian rhythm. Light exposure prior to the onset of sleep can interfere with sleep onset, such as with a child exposed to bright indoor lights, television, or sunlight on long summer days, and light exposure near sleep's end can accelerate awakening. Many other social and environmental cues affect the circadian rhythm, including feedings/mealtimes, ambient temperature, noise, bedtime routines, physical activity, pain, and medications (Zee & Turek, 1999). There is also a circadian rhythm to a myriad of other functions, including endocrine hormone secretion, core body temperature, and sensory processing (Harrington and Mistlberger, 2000 and Mistlberger and Rusak, 2000).

The homeostatic process is the mechanism that drives the body to sleep. This process involves the sleep debt that is accumulated during waking hours, which leads to an increase in the sleep drive. In other words, the longer the period of wakefulness, the stronger the drive to sleep. During a normal schedule of activity during the day and sleep at night, sleep pressure develops during the day and is relieved by daytime naps and nighttime sleep. This process enables the body and mind to rejuvenate and restores alertness (Sack, 2002).

The ultradian rhythm refers to the alternation of two distinct types of sleep—nonrapid eye movement (NREM) and REM sleep—throughout the sleep period. Each type of sleep is associated with distinctive levels of arousal, autonomic response, brain activity, and muscle tone. Based on electroencephalogram (EEG) recordings, NREM sleep has been categorized into four distinct stages. The stages represent gradations in depth of sleep and difficulty of arousal, with stage 1 being the lightest and stage 4 being the deepest (Zee & Turek, 1999). When first drifting off, the child enters stage 1 NREM sleep, which is typified by reduced body movements, drowsiness, and reduced responsiveness. This stage is considered to be a transitional phase between sleep and wakefulness and the child can be easily awakened from this stage of sleep. Stage 1 NREM sleep comprises approximately 2% to 5% of total sleep, with the majority occurring in the beginning of the sleep period (Adair & Bauchner, 1993).

Stage 2 sleep quickly follows stage 1 and is considered the onset of true sleep. Decreased eye movements, reduced muscle tone, and deceleration of respirations and heart rate characterize this stage. The child is able to move freely and reposition in the bed. About half of the total sleep time is spent in stage 2, with the majority occurring in the middle of the night (Adair & Bauchner, 1993).

The third and fourth stages are nearly identical and are collectively called delta, deep, or slow-wave sleep (SWS) (Zee & Turek, 1999). A relaxed body position, slow and rhythmic breathing, and a decreased heart rate characterize these stages. Arousal is difficult and if awakened, the child will appear confused and disoriented. The preponderance of stage 3 and 4 sleep is seen in the early hours of the sleep period, and these stages constitute approximately 20% of total sleep time (Adair & Bauchner, 1993).

REM sleep is characterized by bursts of rapid eye movement, intense EEG activity, muscle paralysis, and dreaming. EEG activity is similar to that of the awake state, suggesting that higher brain functioning is actively involved during the REM sleep period (Anders, Sadeh, & Appareddy, 1995). The muscle paralysis in the presence of intense brain activity seen in REM sleep has led to the synonymous term "paradoxical sleep" (Zee & Turek, 1999). Muscle twitches, facial expressions, and vocalizations related to dream activity are observed. However, the child remains essentially paralyzed, which is thought to protect the sleeper from physically acting out the action in dreams. Dreams are often vividly remembered and frequently depict very active or unusual themes. Frequent changes in respiration and heart rate are also common in REM sleep. REM sleep is thought to be a time for the brain to assimilate images by replaying them during dreams and "learn" from the experiences of the day (Adair & Bauchner, 1993). The proportion of REM sleep is highest in infancy (55%) and declines to about 20% to 25% by age 5 years (Anders et al., 1995).

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