Newborns spend 50% of their sleep time in rapid eye movement (REM) sleep (also known as active sleep or dreaming sleep) and 50% of their sleep time in non-REM (NREM) sleep (also known as quiet sleep or slow-wave sleep). By adolescence, the relative proportion of REM sleep during sleep time has diminished to 20%, whereas NREM sleep has increased proportionally to 80% of sleep time. Four stages of NREM sleep (Stages 1-4) can begin to be differentiated from patterns on the electroencephalogram (EEG) by age 3-4 months.

REM and NREM periods alternate with each other in 50- to 60-minute (i.e., ultradian) sleep cycles. A period of 30 minutes of REM sleep is followed by 30 minutes of NREM sleep in a sleep cycle, and three to four sleep cycles constitute a 4-hour episode of sleep for the newborn. At age 3 months, diurnal influences begin to affect sleep-cycle organization (Ferber 1999).

Early sleep cycles have 40-50 minutes of NREM sleep and 10-20 minutes of REM sleep; in sleep cycles later in the night, the reverse is true. Thus, NREM sleep, especially Stage 4 NREM sleep, shifts to the beginning of a sleep period, and REM sleep predominates later in the sleep period. As the continuous periods of sleep consolidate and lengthen, the number of REM-NREM sleep cycles increases. However, the 50- to 60-minute sleep cycle itself does not lengthen until adolescence, when the 90-minute sleep cycle of mature adults is achieved.

Another noteworthy developmental landmark occurs at sleep onset. When young infants make the transition from wakefulness to sleep at the beginning of their night, their initial sleep-onset state is typically REM sleep. By age 3 months, sleep-onset REM periods begin to be replaced by sleep-onset NREM periods. By the time the child is 1 year old, transitions from waking directly to REM sleep are rare. Appreciating these maturational changes is useful for clinicians in differentiating many of the common sleep disorders that affect infants, children, and adolescents.