Babies wake at night for a combination of biological, developmental, and environmental reasons, and the specific cause depends heavily on your baby’s age. A newborn’s brain simply hasn’t developed the internal clock that distinguishes day from night. Babies don’t begin producing melatonin, the hormone that drives sleepiness in darkness, until around 3 months old. Before that, frequent waking is completely expected.
But even after those first few months, nighttime sleep can fall apart for a surprisingly long list of reasons. Here’s what’s most likely going on and what you can do about it.
Their Internal Clock Isn’t Ready Yet
Adults have a circadian rhythm that makes them sleepy at night and alert during the day. Newborns don’t. Their melatonin production doesn’t kick in until roughly 3 months, and their cortisol cycle, which helps regulate waking, continues developing between 2 and 9 months. So for much of the first year, your baby’s body is still building the biological machinery that makes consolidated nighttime sleep possible.
This means that a newborn waking every two to three hours isn’t a problem to solve. It’s the expected behavior of a brain that hasn’t yet learned what nighttime means. You can help the process along by exposing your baby to natural light during the day and keeping nighttime feeds dim and quiet, but the timeline is largely set by biology.
They’re Hungry (and Their Stomach Is Tiny)
At birth, a baby’s stomach holds about 1 to 2 teaspoons. By day 10, it’s roughly the size of a ping-pong ball, around 2 ounces. That small capacity means your baby digests a feeding quickly and genuinely needs to eat again. This is especially true for breastfed babies, since breast milk digests faster than formula.
Around 6 months, hunger remains one of the most common reasons for nighttime waking. Babies at this age are burning more calories during the day as they practice scooting, crawling, and sitting, and they may need to replenish at night. If your baby is feeding well during the day, taking in enough total volume, and gaining weight on track, nighttime hunger typically decreases gradually without any special intervention.
They’ve Missed Their Sleep Window
This one is counterintuitive: a baby who stays awake too long doesn’t just get sleepier. They get wired. When your baby becomes overtired, their stress response kicks in and floods their body with cortisol and adrenaline. Cortisol disrupts the sleep-wake cycle, and adrenaline triggers a fight-or-flight state. The result is a baby who seems hyperactive, fussy, and paradoxically harder to get to sleep, not easier.
The fix is watching wake windows, the stretch of time your baby can comfortably stay awake between naps. These change significantly with age:
- Birth to 1 month: 30 minutes to 1 hour
- 1 to 3 months: 1 to 2 hours
- 3 to 4 months: 1.25 to 2.5 hours
- 5 to 7 months: 2 to 4 hours
- 7 to 10 months: 2.5 to 4.5 hours
- 10 to 12 months: 3 to 6 hours
If your baby has been awake for longer than their age-appropriate window, especially in the hours before bedtime, overtiredness is a likely culprit. Starting your bedtime routine earlier, even by 15 to 30 minutes, can make a noticeable difference.
Sleep Regressions Are Real
Just when you think you’ve figured things out, your baby’s sleep can suddenly fall apart. Sleep specialists identify roughly six regression windows: at 4, 6, 8, 12, 18, and 24 months. Each one tends to coincide with a specific developmental leap.
The 4-month regression is often the most dramatic because it reflects a permanent change in how your baby’s brain cycles through sleep stages. Before this point, babies drop into deep sleep quickly. Afterward, they cycle through lighter sleep phases like adults do, which means more opportunities to wake up. The 8-month regression often lines up with learning to pull to standing or crawl, plus the arrival of central incisor teeth. At 12 months, growing awareness of the world keeps babies mentally stimulated even when they should be winding down.
The 18-month regression has a strong emotional component. Toddlers at this age often experience separation anxiety and resist being away from their parents at bedtime. By 24 months, causes shift to things like nightmares, nighttime fears, and life changes such as potty training or moving to a toddler bed. Regressions typically last two to six weeks. They’re temporary, even though they don’t feel that way at 3 a.m.
Sleep Associations and Object Permanence
If your baby falls asleep while being rocked, nursed, or held, and then wakes up alone in a crib, the world has changed from their perspective. Once babies develop object permanence (the understanding that things still exist when out of sight, which starts around 4 to 8 months), they notice and care about these differences. They may wake between sleep cycles, realize the rocking or nursing has stopped, and cry to get it back.
This doesn’t mean you need to stop rocking or nursing your baby. It means understanding why they wake up calling for you. If the frequent waking is unsustainable for your family, gradually shifting the association so your baby falls asleep in the crib (drowsy but awake, for example) can reduce the number of times they need you to recreate those conditions overnight.
The Room Itself Might Be the Problem
Small environmental factors add up. The recommended room temperature for infant sleep is 68 to 72°F, regardless of season. A room that’s too warm is one of the most common and easily fixable causes of restless sleep. Dress your baby in one layer more than you’d wear comfortably, and check the back of their neck or chest for dampness. If they’re sweating, they’re too warm.
Light matters too. Even small amounts of light can suppress the melatonin your baby is only just starting to produce. Blackout curtains help, particularly in summer when sunset comes late. Noise is a more individual variable: some babies sleep better with white noise that masks household sounds, while others are fine without it.
Teething: Maybe Less Disruptive Than You Think
Teething gets blamed for a lot of sleepless nights, but the research is surprisingly mixed. A study published in The Journal of Pediatrics that analyzed objective sleep data, including total sleep time and number of awakenings, found no significant differences between teething nights and non-teething nights. Another prospective study of 21 infants found no sleep changes on the day of eruption or the five days before it.
That said, a larger study tracking 231 tooth eruptions did find a bump in sleep disturbance on the day of eruption and the day after. So teething may cause a brief disruption of a day or two, but it’s unlikely to be the explanation for weeks of poor sleep. If your baby’s sleep has been off for longer than a few days, something else is probably contributing.
When Poor Sleep Signals Something Medical
Most nighttime waking is normal and developmental. But some patterns point to a medical issue worth investigating. Reflux, particularly the “silent” type without obvious spitting up, can make lying flat uncomfortable and cause repeated waking. Signs include arching of the back during or after eating, gagging or trouble swallowing, irritability that worsens after feeds, refusing to eat, wheezing, and poor weight gain.
Reflux that’s mild often improves with feeding adjustments like smaller, more frequent meals and keeping your baby upright for 20 to 30 minutes after eating. Medication is typically only considered when a baby has persistent symptoms despite feeding changes and is struggling to gain weight or sleep. If your baby is arching, frequently vomiting, or not gaining weight, that combination is worth bringing up with their provider.
A Safe Sleep Setup for Better Nights
Whatever the cause of your baby’s waking, the sleep environment needs to be safe every time you put them down. The current recommendations call for a firm, flat surface with no incline greater than 10 degrees. That means a crib, bassinet, portable crib, or play yard that meets Consumer Product Safety Commission standards. Baby nests, pods, inclined sleepers, hammocks, and in-bed sleepers do not meet these standards and are not considered safe for sleep.
Swaddling is fine for young newborns placed on their backs, but it needs to stop as soon as your baby shows signs of attempting to roll, which typically happens around 3 to 4 months. A swaddled baby who rolls onto their stomach faces a suffocation risk. Weighted swaddles and weighted sleep products are not recommended at any age.