The Science of Circadian Rhythm Development in Infants

By Birth & Baby University · Last updated: October 2026 · 7 min read

New parents often ask when the baby will “know day from night.” The question sounds simple, and the answer involves several different things that develop on different timelines: the internal body clock, the hormones it controls, the baby’s ability to sleep for longer stretches, and the ability to stay settled through the night. Postpartum doulas get this question in the first weeks, usually at 3 a.m. This article looks at what published reviews and studies say about when circadian rhythms emerge, how nighttime sleep consolidates over the first year, and where the claims popular in parenting advice, such as light exposure routines, outrun the evidence.

Key takeaways

Quick answer: the circadian system begins developing before birth, but pronounced day-night rhythms in sleep and hormones generally appear after about two months, and sleep consolidates most between birth and four months. Studies differ on timing: one longitudinal study found a salivary cortisol rhythm already at one month. Babies vary a great deal, and no routine has been shown to make a newborn “get” day and night faster.

• A pediatric review states that after birth there is progressive maturation of the circadian system, with pronounced rhythms in sleep-wake and hormone secretion generally developing after 2 months of age.
• A longitudinal study of 130 healthy full-term infants found a significant group-level salivary cortisol rhythm already at one month, earlier than previous studies, with considerable individual variability.
• A review of infants’ nocturnal sleep consolidation found the most marked changes in the first four months, especially at 1 and 2 months, with minimal change through 9 months, and that most infants can sleep 8 hours by age 6 months.
• A systematic review of 34 studies gave a reference mean total sleep duration for infants of 12.8 hours (range 9.7 to 15.9) and 0 to 3.4 night wakings per night for infants aged 0 to 2 months.

What the research says

Rhythms begin early and mature gradually. Rivkees’ 2003 review in Pediatrics summarized the evidence that the circadian timing system develops prenatally and that the body’s clock in the brain’s suprachiasmatic nuclei is present by midgestation in primates. It reported that the circadian system of primate infants responds to light at very premature stages, that after birth there is progressive maturation, with pronounced sleep-wake and hormone rhythms generally appearing after 2 months of age, and that premature infants exposed to low-intensity cycled lighting established rest-activity patterns in phase with the 24-hour cycle earlier. That finding comes from premature infants, so it should not be treated as proof that lighting routines speed up term newborns. A later longitudinal study by Ivars and colleagues (2015) measured salivary cortisol in 130 healthy full-term infants in the morning, noon and evening each month from birth to 12 months. They found a significant group-level circadian rhythm at one month that remained throughout the first year, with considerable individual variability, and no correlation with parents’ reports of behavioral regularity or exposure to traumatic events. The result is earlier than previous studies suggested. In other words, the hormone rhythm may be present sooner than the sleep-wake rhythm that parents see, which is part of why sources disagree about when the clock “starts.”

Melatonin and feeding. The newborn’s pineal gland does not produce melatonin rhythmically for several months. Häusler and colleagues’ 2024 review notes that human milk is the main natural source of melatonin for breastfed newborns, with diurnal changes and a peak early after midnight, and proposes that synchronized feeding could aid circadian entrainment. The authors state that factors affecting melatonin content in milk and their clinical significance are understudied. This is a hypothesis-generating narrative review, not evidence that a feeding time or milk-timing practice improves a baby’s sleep, and it should not be presented to parents as a recommendation.

What parents will notice: consolidation. Henderson, France and Blampied’s 2011 review of the development of infants’ sustained sleep found clear trends despite methodological differences between studies. The most marked change in longest sustained sleep, longest self-regulated sleep and sleeping through the night happened across the first four months, particularly at ages 1 and 2 months, with minimal change to 9 months and a small increase thereafter. The authors argue that an older definition of sleeping through the night (midnight to 5 a.m.) may have underestimated infants’ capacity, and that most infants can sleep 8 hours by 6 months and 9 or more hours afterward. A broader review by Galland and colleagues (2012) pooled 34 studies of diary and questionnaire data. They reported a mean infant sleep duration of 12.8 hours (range 9.7 to 15.9), 0 to 3.4 night wakings a night for infants aged 0 to 2 months, and that sleep was about an hour shorter in studies from predominantly Asian countries, concluding that values should be treated as global norms combining cultures. Parent-reported sleep is imprecise, and the age bands are wide.

What this means for doulas

• Set expectations kindly. “The body clock is developing, and many babies start showing clearer day and night patterns somewhere after two months, though they vary” is consistent with the reviews. Avoid promising a date.
• Present light routines as reasonable, not proven. Bright, ordinary daytime activity and dim, quiet nights are low-risk and match how the clock responds to light in the premature-infant research, but no source reviewed here tested them in healthy term newborns at home.
• Respect feeding needs. Newborns wake at night to feed, and feeding frequency is a clinical matter. Do not encourage dropping night feeds in the early weeks. See Why Newborns Wake Frequently at Night: The Biology of Infant Sleep.
• Connect to sleep cycles and safety. The structure of newborn sleep is covered in Understanding Newborn Sleep Cycles, and every sleep setup should follow Safe Sleep Recommendations: What Every Postpartum Doula Must Know.
• Normalize variation and ask about exhaustion. Range matters more than the mean. If parents are exhausted or distressed, help them plan rest and consider whether their own clinician should be involved.

Putting the evidence into practice

1. Ask what a typical day and night look like. Keep a three-day log of sleep and feeds if the parent wants one.
2. Offer a gentle day-night rhythm. Open curtains in the day, keep nighttime feeds calm and dimly lit, and avoid stimulating play at night. Frame these as comfortable habits, not a training program.
3. Do not use schedules to override cues. A baby who is hungry or hard to settle needs a response regardless of the clock.
4. Make nights survivable. Practical help such as a night shift, protected sleep blocks and shared feeds may do more for the family than any attempt to shape the baby’s rhythm.
5. Refer. A baby who sleeps unusually long without waking to feed, is hard to rouse, or has unusual breathing during sleep needs the pediatric provider promptly.

Limits and uncertainty

The studies reviewed here disagree on when a rhythm appears, because they measure different things (hormones, sleep-wake behavior, parent reports) in different samples. The 2003 review includes animal and premature-infant evidence and is now more than two decades old; the cortisol study comes from a single research group and is group-level; the melatonin paper is a narrative review that calls its own topic understudied; and sleep norms rely on parent-reported diaries with wide age bands. This article draws on abstracts, not full texts. None of these sources tested routines for helping newborns adapt to day and night, so such routines remain common-sense suggestions. Premature babies, babies with medical conditions and babies in the NICU follow different timelines.

The bottom line

An infant’s body clock develops over the first months, with sleep consolidating most between birth and four months and clearer day-night patterns generally following after about two months, though hormone rhythms may appear sooner and every baby is different. Evidence for any specific parent-led routine is thin. A doula can set realistic expectations, support calm nights and daytime light as comfortable habits, protect the family’s rest and keep safe sleep non-negotiable.

Birth & Baby University’s For Doulas page outlines its postpartum doula training, recertification and mentorship for doulas who support families through newborn sleep and the early weeks. Confirm that any program meets your certifier’s, employer’s or state’s requirements before you enroll.

This article is for professional education. It is not medical, legal or tax advice. Check the standards of your certifying organization and the rules where you work. For concerns about a baby’s health, contact a pediatric provider or emergency services.

Sources and further reading

• Rivkees SA. Developing circadian rhythmicity in infants. Pediatrics (2003). Read the source
• Ivars K, et al. Development of salivary cortisol circadian rhythm and reference intervals in full-term infants. PLOS ONE (2015). Read the source
• Häusler S, et al. Melatonin in human breast milk and its potential role in circadian entrainment: a nod towards chrononutrition? Nutrients (2024). Read the source
• Henderson JMT, France KG, Blampied NM. The consolidation of infants’ nocturnal sleep across the first year of life. Sleep Medicine Reviews (2011). Read the source
• Galland BC, et al. Normal sleep patterns in infants and children: a systematic review of observational studies. Sleep Medicine Reviews (2012). Read the source

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