The Science Behind the Sleep of Newborns and Infants
Hint: especially useful for new parents!
When a new baby arrives, one of the biggest adjustments for parents is adapting to their little one's sleep patterns and needs. Sleep is absolutely vital for an infant's growth, development, and well-being, playing a crucial role in brain maturation, physical growth, and immune system functioning. However, an infant's sleep cycles and habits are vastly different from those of children and adults. Understanding these differences is key for tired new parents struggling to establish healthy sleep routines.
Before birth, fetuses rely solely on maternal circadian signals like melatonin, cortisol, and body temperature rhythms to entrain their developing circadian clocks in utero. Circadian clocks are internal biological timers that regulate the sleep-wake cycle and other physiological processes over a roughly 24-hour period. These "body clocks" synchronize to external cues like light and dark cycles.
The fetal suprachiasmatic nucleus (SCN), a part of the brain which controls circadian rhythms, exhibits oscillations (activity) by the third trimester, though at lower amplitudes than adults, indicating dependence on maternal entrainment. Melatonin from the mother easily crosses the placenta, and its cyclical pattern helps synchronize the fetal circadian clock via melatonin receptors present at 18 weeks of gestation. This reliance on maternal time cues highlights how the fetal circadian system develops in conjunction with the mother's rhythms before birth.
At birth, infants abruptly lose the maternal circadian cues that helped synchronize their biological clocks in utero. With this loss of the mother's signals like melatonin and temperature cycles, newborns must now rapidly establish their own independent circadian rhythms driven primarily by light/dark exposure. The newborn SCN has yet to fully mature and entrain to the 24-hour day/night cycle. Breast milk can provide beneficial transitional support, as it contains low residual levels of maternal circadian signals like melatonin and cortisol that may help calibrate the infant's rhythms initially. Remember, newborns for the first few weeks cannot produce melatonin or cortisol so they rely exclusively on external sources like breast milk. Studies show breastfed infants tend to develop more consolidated circadian sleep/wake cycles earlier than formula-fed infants.
By week 5 after birth, newborns develop a 25-hour circadian rhythm (so called “infra-dian” rhythm since it lasts longer than one full day) and sleep in about 4h intervals. At about 4 months of age, more consolidated wake and sleep episodes appear, and by 6 to 9 months of age most infants are able to sleep through the night, displaying at least 6h consolidated sleep episodes. Over these first few months, the infant's SCN becomes increasingly synchronized to environmental light/dark patterns. Daytime light exposure triggers specialized photoreceptive neurons in the retina in the eye to signal the SCN (just like in adults), kicking off a cascade of neurochemical processes that align the clock's oscillations to a 24-hour rhythm. Providing newborns with time-of-day cues through regular light exposure, feedings, and interactions can help this innate programming of the circadian clock develop appropriately. With a strong circadian system in place, infants can better regulate their sleep/wake patterns and other biological functions.
When the circadian clock is just starting to develop, newborns sleep a majority of the day, averaging around 16-18 hours of sleep over a 24-hour period. However, as mentioned earlier, those hours are broken up into shorter sleep periods of only about 2-4 hours at a time. Much of a newborn's sleep occurs in the rapid eye movement (REM) sleep stage, when most dreaming takes place. Amazingly, full-term newborns can spend up to 50% of their sleep in REM, compared to only 20-25% for adults. It is believed that this excessive REM sleep plays a vital role in a newborn's brain development and growth in those first few months.
Infants have very short sleep cycles that last only around 50 minutes, compared to the 90-100-minute cycles for adults. These abbreviated cycles mean babies transition between the different sleep stages (e.g. deep sleep or REM) more frequently throughout the night, which can result in more nighttime awakenings and disruptions. It's common for newborns to wake up 5-6 times per night initially.
As infants grow over the first 6 months of life, their sleep patterns gradually evolve to start resembling more typical circadian cycles. Daytime sleep periods grow shorter while nighttime sleep stretches get longer, with many infants sleeping around 9-12 consecutive hours per night by 6 months old. The number of daytime naps also decreases, with most infants transitioning to taking just 1-3 naps per day totaling 2-4 hours by this age.
These early months of fragmented sleep and frequent night wakings, while undoubtedly exhausting for parents, are simply biological necessity for babies. The infant's irregular sleep patterns result from their immature circadian systems that have yet to fully entrain. With consistent routines, proper time cues, and patience, these natural sleep rhythms will inevitably mature and consolidate.
New parents should understand that fractured sleep is typical infant behavior. Accommodating these distinct sleep needs allows babies' circadian clocks to properly set while preventing excessive fatigue and stress. Though the adjustment is taxing, this irregular newborn sleep is temporary. Recognizing it as an important developmental phase can help caregivers cope better with this precious but challenging time.
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About the author
Alen Juginović is a medical doctor and postdoctoral researcher in the Department of Neurobiology at Harvard Medical School studying the effect of poor sleep quality on health. He is public and keynote speaker and teacher at Harvard College. He and his team also organize international award-winning projects such as conferences which attracted 2400+ participants from 30+ countries, 10 Nobel laureates and major leaders in medicine (Plexus Conference), collaborative research projects, charity concerts and other events. He co-founded Med&X Association, a non-profit organization that organizes conferences with Nobel laureates and partners with leading universities and hospitals around the world to help accelerate the development of talented medical students and professionals. Feel free to contact Alen via LinkedIn for any inquiries.
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