It isn’t always easy to look at a baby and see the adult she is going to grow into, but in many ways, babies’ sleep patterns aren’t actually that different from their parents’. You may not know whether your newborn’s squishy little face has your nose or your grandmother’s eyebrows yet, but it’s a safe bet that, just like yours, her sleep cycles between REM and non-REM sleep, and that there are four phases of non-REM sleep she goes through. Still, there’s a reason new parents are known for being short on sleep - newborn sleep patterns still have a ways to go before they mature.
Though it can be entirely difficult for an adult to swtich to follow along with a newborn's sleep patterns, the cycles of sleep infants and adults go through actually have a lot of similarities. In both newborns and adults, as well as everyone in between, the first part of the cycle is non-REM (rapid eye movement) sleep, of which there are four stages. These four stages move the body from the beginnings of drowsiness through to a deep sleep. The stage after non-REM sleep is REM sleep, which is named for the rapid eye movements that happen as dreams occur in this lighter stage of sleep. During REM sleep, brain waves look similar to waking brain waves, breathing picks up, and the body is temporarily paralyzed. The cycle of non-REM and REM sleep often repeats itself several times, getting longer each time, and in adults, these cycles often repeat as many as many as four or five times in a night.
The human body has two general forces towards sleep, and those are circadian rhythms and sleep drive. Circadian rhythms are your “mental clock,” and are related to the way the body responds to light cues, temperature and routines. They help to dictate sleeping, waking, and eating patterns, and one of the major ways they do so is by cueing the body to start producing melatonin, a sleep hormone, at night.
Sleep drive, on the other hand, is basically how tired you are. Just like the need for food or oxygen, the need for sleep builds up over time, although the need for sleep generally takes longer than the drive towards food, and definitely doesn’t happen nearly as quickly as the drive for oxygen between one breath and the next. Before babies’ circadian rhythms develop, which can take weeks or months, it’s sleep drive that pushes them into all those little cat-naps, just like how it’s often hunger drive that wakes them up again. As babies get older, they grow more and more able to wait longer between sleep times, and their bodies start to develop circadian rhythms based on a 24-hour clock, instead. Parents start to build the foundations for circadian rhythms as they start to encourage longer stretches of sleep at night, and shorter naps during the day.
Aside from the way babies aren’t born with circadian rhythms, one of the biggest differences between infant and adult sleep patterns is just that babies’ sleep cycles are shorter. This gives them the ability to move through each of the stages of sleep during shorter stretches of sleep, like brief naps.
Babies sleep more than adults, too (though it may not feel like it, with how often they wake up), but the added sleep has more to do with what their brains are doing during sleep than with the process of sleep itself. In adults, sleep is used to conserve energy, repair muscle, clear out waste, and process the memories formed during the day. In babies, sleep does all of that, and then a bit more.
Although sleep was once thought of as a passive state, it’s now known to be a state where the brain processes the experiences of the day, and the body regains energy. Sleep plays a big role in the way the brain processes experiences and learns from them, and stores memories. Since the whole world is new to a newborn, there are a lot more new experiences to process, learn from, store, and remember every day than there are for adults. Sleep is also the time when the body releases Human Growth Hormone, which plays a role in the way newborns grow fast enough to double their birth weight by 5 months old.
It’s thought that one of the reasons why babies need so much sleep early on is because they’re learning so much every moment they’re awake. Sleep has been shown to help with brain plasticity, or the brain’s ability to process the new information it’s taking in, and remember it later. If sleep helps babies process information and memories, it helps babies get the most out of the experiences they have every moment they’re awake - even early on, when moments they’re awake are few and far between.