Newborns sleep better during the day because they haven’t developed their own internal clock yet. In the womb, your baby relied entirely on your hormones to distinguish day from night. Once born, that signal disappears, and it takes roughly 2 to 3 months before a baby begins producing the hormones that regulate a consistent sleep-wake cycle. Until then, many newborns default to longer, deeper stretches of sleep during daylight hours and more wakefulness at night.
Why Newborns Don’t Know Day From Night
During pregnancy, your melatonin (the hormone that signals darkness and sleepiness) crossed the placenta and gave your baby a borrowed sense of time. That transfer stopped at birth. Newborns are born without a circadian rhythm in cortisol secretion either. Research estimates that infants first establish a recognizable cortisol pattern somewhere between 2 and 3 months of age, though for some babies it takes as long as 9 months. At one month old, only a small minority of infants show lower cortisol in the evening and higher levels in the morning, the pattern adults rely on to feel alert during the day and sleepy at night.
Without these internal signals, a newborn’s sleep is essentially random. Their sleep cycles are short (around 45 to 60 minutes), and roughly half of all newborn sleep is active sleep, the infant equivalent of REM. During active sleep, babies twitch, move their arms and legs, breathe irregularly, and wake more easily. Because these cycles repeat around the clock with no hormonal bias toward nighttime, the result often looks like a baby who naps beautifully in bright afternoon light but is wide-eyed at 2 a.m.
How Breast Milk Plays a Role
If you’re breastfeeding, your milk actually contains different hormones depending on the time of day. Nighttime breast milk has significantly higher melatonin levels than daytime milk. In one widely cited study, daytime milk had melatonin levels so low they were undetectable, while nighttime milk contained measurable concentrations. Morning milk, meanwhile, contains higher levels of cortisol, the hormone associated with alertness. This pattern holds true across all stages of lactation, from colostrum in the first days to mature milk weeks later, regardless of whether the baby was born early or on time.
This hormonal variation in breast milk acts as one of the first external cues that helps a newborn begin to distinguish day from night. The interplay between cortisol peaking in morning milk and melatonin peaking in nighttime milk may help coordinate sleep and alertness phases in a newborn. It’s one reason lactation experts recommend feeding pumped milk at roughly the same time of day it was expressed, so the hormonal signals match the clock.
Daytime Environments Can Help Babies Sleep
There’s also a simpler explanation many parents overlook: daytime environments are often unintentionally ideal for newborn sleep. The background hum of a household, conversations, traffic noise, and general activity creates a steady wash of sound similar to what your baby heard in the womb. Newborns respond to wider-band noise (sound spread across many frequencies, like a fan or ambient household noise) more readily than narrow, isolated sounds. That consistent daytime backdrop can actually lull a newborn into deeper sleep.
At night, by contrast, the house goes quiet. Every small sound stands out. You’re also more likely to be in a dark, still room where a baby’s short sleep cycles mean they wake fully between cycles rather than drifting back to sleep. The irony is that the conditions parents create for “good” nighttime sleep, silence and darkness, can work against a newborn who hasn’t yet learned what those cues mean.
When This Pattern Starts to Shift
Most babies begin showing a preference for nighttime sleep between 6 and 12 weeks of age, as their own melatonin production comes online and their cortisol rhythm starts to emerge. This process is gradual. You won’t see a sudden flip from day sleeping to night sleeping. Instead, you’ll notice one longer stretch of nighttime sleep that slowly extends over weeks.
The shift doesn’t happen on its own, though. Light exposure is one of the strongest signals you can give your baby’s developing clock. Research on infant circadian development recommends daytime light levels between 100 and 200 lux, with some exposure to natural light, and keeping nighttime artificial light below 50 lux. In practical terms, this means letting your baby experience bright, naturally lit rooms during the day, even during naps, and keeping nighttime feedings and diaper changes as dim and boring as possible.
What You Can Do Now
You won’t override your newborn’s biology, but you can reinforce the signals that help their internal clock develop on schedule.
- Maximize daytime light contrast. Open curtains and blinds during the day. Take your baby near windows or outside when weather permits. The goal is consistent bright exposure during waking hours, starting in the morning. Studies on cycled lighting for infants typically begin light exposure around 6:00 or 7:00 a.m.
- Keep nights dark and low-key. Use the dimmest light you can manage for nighttime feedings. Avoid screens and overhead lights. Don’t play, talk much, or stimulate your baby during overnight wake-ups.
- Don’t fight daytime naps. It’s tempting to keep your baby awake during the day hoping they’ll sleep better at night. This backfires. Overtired newborns sleep worse, not better. Let them nap freely while using light and activity to mark daytime as “different” from night.
- Time pumped milk if possible. If you’re pumping and bottle-feeding breast milk, labeling it with the time it was expressed and offering it at a similar time of day preserves the natural melatonin and cortisol signals your milk carries.
The day-night confusion you’re experiencing is one of the most universal challenges of the newborn period, and it’s rooted in biology, not anything you’re doing wrong. Your baby’s brain is building its internal clock from scratch, using every light cue, feeding signal, and environmental pattern you provide. For most families, the worst of it resolves by 3 months, often sooner.