Sleep is a universal phenomenon that is critical to child health, development, and daily functioning. Expectations and habits surrounding sleep vary greatly and must be interpreted in the context of each family and culture. Sleep is broadly categorized by polysomnographic patterns into rapid eye movement (REM) sleep and non-REM (NREM) sleep. REM sleep is characterized by an active, awake-like electroencephalography (EEG) pattern and muscle atonia. NREM sleep is further divided into three stages. Stage 1 (N1) is the lightest sleep stage and consists of low-amplitude, high-frequency EEG activity. Stage 2 (N2) is similar to Stage 1 although eye movements stop. Brain waves become slower with occasional bursts of rapid waves (called sleep spindles or K complexes ). Stage 3 (N3), also known as deep or slow-wave sleep, is characterized by low-frequency, high-amplitude delta waves. REM and NREM sleep alternate in cycles throughout the night.

Sleep architecture changes from fetal life through infancy and childhood and parallels physical maturation and development. Newborns require the most sleep and have the most disrupted sleep patterns; newborn sleep cycles last approximately 60 minutes and gradually lengthen to 90 minutes in children and adults. Neonates typically begin their sleep cycle in REM sleep, whereas older children and adults begin sleep in NREM sleep. REM sleep comprises up to 50% of total sleep time in newborns and gradually decreases to 25-30% by adolescence. Slow-wave sleep is not seen before 3-6 months of age. Starting at 6-12 months and continuing through adulthood, the amount of REM sleep shifts toward the last third of the night, while NREM sleep predominates during the first third of the night. Infants are capable of sleeping through the night without feeds at around 6 months of age; this age varies with many factors including gestational age, type of feeding (e.g., breast-fed infants tend to awaken more frequently than formula-fed infants), familial factors, and general cultural contexts.

The timing and duration of sleep also change with age. Sleep duration gradually decreases with age (0-12 years), while longest sleep period increases (0-2 years); number of night awakenings decrease in the first 2 years of life; number of daytime naps decreases (up to age 2). Sleep patterns also become more diurnal. Full-term infants sleep on average 16-18 hours per day in fragmented intervals throughout the day and night, and 1- to 3-year-old children sleep on average 10-16 hours. Naps begin to decrease from two naps to one starting around 15-18 months of age. The biological need for napping diminishes after age 2; significant consistent napping after age 2 has been associated with later sleep onset and reduced sleep quality and duration. However, 50% of 3-year-olds still nap. Napping in the older child and adolescent suggests insufficient sleep or a sleep disorder.