Infant Sleep Biology
Infant Sleep Biology
Sleep Biology: Why we need it and how we do it
Babies sleep more than adults and without the day-night pattern (circadian rhythm) that adults typically have with their sleep.
How do humans sleep?
Sleep architecture, the pattern of sleep cycles and different phases of sleep throughout the night, changes from infancy to adulthood.
Adults have sleep cycles through the night, from initial deep sleep early in the night to rapid eye movement (REM) phases in the second half of the night (Sanchez et al., 2022). In contrast, newborns have shorter sleep cycles and more frequent wakings (Ball, 2025). Infants have four broad categories of sleep given they are not yet mature:
Sleep Development (Consolidation)
It’s completely normal for newborns to wake between sleep cycles, sometimes after just one, sometimes after two or three strung together (Ball, 2025). As their brains mature, some babies begin to experience what’s known as sleep consolidation, where they stay asleep through multiple sleep cycles without waking.
You may hear that babies should start sleeping in longer stretches around three to four months, but research shows this sleep consolidation only happens in about 1 in 3 babies (Ball, 2025). The majority will still wake between sleep cycles, day and night.
Even babies who begin to sleep for longer periods early on may start waking again later. This is often due to normal developmental changes, not a sign that something is wrong. While some people refer to this as a sleep regression, it’s important to know that this term has no biological basis — it’s simply part of how babies grow and learn (Ball, 2025).
Tip: Around four months of age babies circadian rhythm will start to develop (with starts and stops). You can support your infant’s sleep consolidation by regularly exposing them to daylight during the day (Iwata et al., 2017).
Sleep Pressure: When do Infants Fall Asleep
When adults sleep is influenced by their circadian rhythm, or body clock, which prompts sleep at night and wakefulness during the day. Babies do not have their own body clock. Circadian rhythms do not begin to develop until around two-four months of age (Sheldon, 2002) but they are not fully established until at least twelve months, but often later (Davis et al., 2004; Anders et al., 1995). The urge to sleep that rises the longer an individual stays awake and reduces after sleep is called ‘sleep pressure’ (Ball, 2025). An adult, or an older child with an established circadian rhythm, will get sleepier throughout the day as their sleep pressure rises, with it peaking in the evening (Ball, 2025). Babies sleep pressure rises differently to adults
Infant Sleep Pressure
Infants will sleep a sufficient amount for their development (Ball, 2025). They will sleep both day and night as they need. To help your baby build up enough sleep pressure during the day so they sleep better at night, let them nap in and around typical day-time activity (such as in well-lit environments and/or with on-going conversations around). Doing this means that your baby won’t sleep as deeply as they would in a dark, quiet room, which helps to build their sleep pressure higher towards end of day (Ball, 2025). Baby’s first sleep cycle upon falling asleep upon going to bed at night tends to be the longest.
Top Tip: Align baby’s bed time with your own so you sync your longest initial sleep period together. Even if that means baby goes to bed a bit later or you go to bed a bit earlier!
Working with and against sleep pressure
Consider what helps your baby relax and have a handful of options you can implement if you are trying to help your tired infant fall asleep. Often, physical contact and soothing touch works for most children (Ball, 2025). If barriers preventing sleep are removed and sleep pressure is high, babies will fall sleep.
What if my baby doesn’t want to fall asleep or isn’t falling asleep as scheduled? This could be an example of trying to prompt your baby to sleep when their sleep pressure is low. Given that babies’ sleep patterns change frequently as they age and biological sleep development progresses, applying rigid sleep timings can be a exhausting and demoralising exercise (Ball, 2025). Innate biological regulators will ensure infants sleep a significant amount when they need it.
Top Tip: If your baby doesn’t seem interested in sleep, consider trying something stimulating such as going for a walk with baby, playing with baby, listening to music with baby, singing nursery rhymes, doing shadow puppets, or showing images such as sensory cards. The activity may vary, but the aim is to interact with your baby to engage their brain.
Infant Sleep Variation
Infant biological regulation naturally supports infants sleeping a sufficient amount. It is important for parents to know that there is a lot of variation between how much any individual baby sleeps. The reason for this variation is difficult to assess and poorly understood, but be reassured no research has established long-term ill-effects for your child if as an infant they slept fewer hours (Ball, 2025).
Sleep is a developmental process, and our sleep needs change throughout our lifetimes. Babies’ sleep patterns mature over the first several years of life, and the sleep patterns of newborns are very different to that of adults. During the first year overall sleep duration falls to around 15 hours, and the majority of sleep becomes consolidated during night-time as circadian rhythms develop (Parmelee et al., 1964).
Popular sleep charts are based on averages of infant sleep which don’t capture the variation. Be reassured, infant sleep lengths vary significantly around these averages.
Sleep and Feeding: Breastfeeding and Formula
The type of milk a baby is fed can influence their sleep patterns. However, the relationship is more complex than it might first appear.
Breastfed vs. Formula-Fed Sleep Patterns
Babies who are breastfed tend to wake more often during the night and have shorter sleep bouts (Elias et al., 1986; Tikotzky et al., 2009). This is partly because breastmilk is easier to digest, leading to more frequent feeds day and night (Anuntaseree et al., 2007; Barnard, 1999). Formula, especially formula derived from cow’s milk, takes longer to digest and has been linked to longer and deeper sleep periods earlier in life (Cavall, 1981; Horne et al., 2004).
Although total night wakings differ, research suggests there’s no significant difference in overall sleep duration between breastfed and formula-fed infants (Quillian, 1997). In fact, breastfeeding mothers and babies often get the same amount of sleep or more overall (Doan et al., 2007; Montgomery, et al., 2010). This is because breastfeeding is more efficient at night: babies and mothers don’t need to fully wake to feed and often fall back asleep quickly, supported by the calming effects of hormones like oxytocin, prolactin, and melatonin found in breastmilk. Melatonin helps babies fall asleep and calms their digestive system, potentially reducing colic or irritability (Engler et al., 2012). Many places sell products claiming to boost sleep hormones like melatonin. These are not suitable for infants.
Waking and SUDI Risk
Formula-fed babies have been found to arouse less easily from sleep than breastfeeding babies (Horne et al., 2004). Waking (arousal) is an important protective mechanism against Sudden Unexplained Death in Infancy (SUDI) (Galland and Elder, 2014). Breastfeeding for two months or more has been shown to lower the risk of SUDI (Vennemann et al., 2009; Kanits et al., 2021).
When parents introduce formula hoping to help their baby “settle” or “sleep through”, this can disrupt the natural breastfeeding process, especially since nighttime feeds are key to maintaining milk supply (due to the role of prolactin, a hormone produced more at night). It may also reduce the protective arousability of infants who are naturally less responsive (Rudzik and Ball, 2016).
Night Waking is Normal
The link between sleep and feeding often reflects cultural expectations. In the UK, breastfeeding is commonly associated with poor infant sleep, and mothers are frequently advised by peers or family to switch to formula or solids to improve sleep (Rudzik and Ball, 2016). However, breastfeeding mothers are more likely to view fragmented sleep as normal, while formula-feeding mothers often see night waking as a problem that needs solving.
In older infants (6–12 months), sleep differences based on feeding method appear to diminish. One study found no difference in night waking or feeding frequency between breastfed and formula-fed babies in this age group (Brown and Harries, 2015). Babies who consumed more calories during the day were less likely to feed at night, but not less likely to wake—highlighting that night waking is a normal part of infant development (St James-Roberts et al., 2015).
Understanding how feeding influences infant sleep can help parents make informed decisions that work best for their family—while also appreciating that frequent night waking is a biologically normal part of infancy.
Last Reviewed: July 2025