We've seen that the cyclic changes in brain activity during adult sleep are quite complex: how and when does an infant establish such a complicated pattern of activity? The answer: regular patterns of sleep-wake activity are established before an infant is even born. Below, we see that the development of regular sleep-wake cycles occurs in stages in the developing infant.
- 26 - 28 weeks gestational age (about 6 1/2 - 7 months): Regular sleep cycles begin in utero. Before this age, unpatterned EEGs indicate irregular sleep-wake cycles. Developing infants now begin to cycle through quiet or non-REM (NREM) and, later, REM stages, two "adult" types of sleep. As in adults, REM sleep follows slow wave NREM sleep. Infants also exhibit "quiet awake" periods of wakefulness characterized by low activity.
- 28 - 30 weeks gestational age (about 7 - 7 1/2 months): REM sleep begins and occupies the majority of sleep. REM and NREM sleep are still interspersed with periods of quiet wakefulness. REM sleep, a period of maximum electrical activity in the brain, is vital for the development of sensory, motor, memory, and learning circuits in the brain: thus it is vital that during this period of rapid neural development before birth most sleep is devoted to REM.
- 40 weeks gestational age (about 9 - 9 1/2 months): REM and NREM sleep comprise approximately equal portions of the sleep cycle. Sleep cycles and EEG patterns are continuous and regular.
- 5 - 9 months after birth: REM sleep is decreased to roughly 20% of the sleep cycle by 8-9 months of age; by 5-8 months infants exhibit nearly adult patterns of sleep.