After the first 90 minutes of gradually descending into non-REM sleep, your brain begins to arouse and move into a lighter and more active kind of sleep, the state of REM sleep. During REM sleep the brain is quite active (it’s when you dream), although the rest of your body is usually relaxed and relatively quiet. You experience rapid eye movement even though your eyes are closed (hence the term REM sleep), and men can get erections. During REM sleep, facial muscles may twitch, producing “sleep grins”. It’s fun to watch for this in babies. Since this is the lightest stage of sleep, it is easiest to waken out of REM sleep.
Adults cycle through REM and non-REM sleep approximately every 90 minutes. Early in the night the periods of non-REM sleep may last as long as 60 minutes, and REM periods may last from 10 to 30 minutes. Toward morning the proportions of non-REM and REM reverse, so that much of early-morning sleep is REM. The length and pattern of these sleep cycles varies greatly between individuals and at different ages. However, during an average eight-hour sleep adults may spend two hours in REM, or active (light) sleep, and six hours in non-REM, or quiet (deep) sleep.
Both of these states of sleep are important for a person’s overall well-being. Non-REM, or deep sleep, is necessary to help the body rest and recuperate. It is known as the restorative state of sleep. REM sleep is necessary for brain development. Understanding these sleep cycles explains why human babies awaken so easily and why it may not be wise to fiddle around too much with babies’ natural sleep cycles.
How babies sleep. Why do babies wake up so much? This is probably the question new parents ask most. The simple answer: because they’re babies. Babies sleep differently from adults.
Babies go to sleep differently. Infants take longer (at least 20 minutes) to drift off and enter deep sleep. On the other hand, adults and older children “crash” into deep sleep, drifting into non-REM sleep in just a few minutes. The younger the infant, the longer it takes him to drift into deep sleep.
What does this sleep fact mean to parents? Babies awaken easily during this drifting off period. Parents don’t have to be sleep scientists to figure this out. Many parents describe their baby as “difficult-to-settle”, or they say “she has to be fully asleep before I can put her down”. Many parents have had the experience where they think their baby is asleep, so they gently carry her to her cot and lay her down – but she wakes up as soon as Mum or Dad turns to tiptoe out of the room. Baby is not truly asleep until he arrives in the state of deep sleep, 20 to 30 minutes after closing his eyes. Trying to hasten the bedtime routine can leave parents very frustrated.
You can see why the advice from sleep trainers to “put babies down in their cots awake” doesn’t work, especially for babies less than three months old. Babies need to be gentled through this first period of REM sleep, so that they can stay asleep until deeper sleep overtakes them. Between three and six months babies begin to drift more quickly into non-REM sleep. They can be put down awake, or partially awake, and they will enter deep sleep fairly quickly.
Bottom line: babies need to be patiently parented to sleep, not just put to sleep.
Babies stay asleep differently. While adults cycle from deep to light sleep approximately every hour and a half, infants move through these states every hour. The younger the infant, the shorter the sleep cycle. What does this mean for parents? When passing from one state of sleep to another, the brain is more likely to awaken than at other times. We call this the “vulnerable period”. If by chance an arousal stimulus (teething pain, loud noise, hunger, separation anxiety, and so on) bothers baby during this vulnerable period, baby is likely to awaken. Because babies have shorter sleep cycles, they have more vulnerable periods – more times during the night when they are likely to wake up. In addition, babies spend more time in REM (light) sleep in the second half of the night. This explains why babies often wake up more during that time.
Bottom line: minimize arousal stimuli during vulnerable periods for night waking.
As babies grow, their sleep cycles lengthen and the percentage of deep sleep increases. There are fewer vulnerable periods during the night when they can awaken easily. They also sleep more deeply and they stay asleep longer – a sleep maturity milestone called settling. The age at which babies settle varies greatly according to the sleep temperament of the baby. The good news is that all babies eventually settle.
Babies’ developing sleep patterns are much like their changing feeding patterns. In the early months babies take small, frequent feedings and short, frequent naps. About fifty per cent of the total sleep of a newborn is REM sleep. This percentage is even higher in premature infants. As you can see from the graph below, as babies grow, they learn to sleep and feed more like adults. These five things happen:
REM (active) sleep decreases
Non-REM (deep) sleep increases
Sleep cycles lengthen
Vulnerable periods for night waking occur less frequently
The total number of hours of daily sleep lessens.
This is called sleep maturity.
Babies are designed this way. Why are babies’ sleep patterns so different from adults’? Answer: because babies need to sleep this way. How babies sleep is one of many things throughout infancy and childhood that parents can’t control, and it may even be unsafe and unwise to try to change. Keep in mind that babies sleep the way they do – or don’t – because they are designed that way for both developmental and survival benefits.
Babies sleep smarter. REM sleep is more than an annoying nuisance that keeps parents as well as babies from sleeping more deeply. The fact that babies’ developing brains don’t turn themselves off as well during sleep as adult brains has developmental benefits. Sleep researchers believe that REM sleep stimulates the infant brain at a time when it is growing very rapidly. Blood flow to the brain increases during REM sleep. The lower brain centres fire off electrical stimuli toward higher brain centres. This stimulation works like mental exercise to help the brain centres develop. The mental activity of dreaming helps the brain grow more neurons. This theory that REM sleep stimulates brain growth is supported by the fact that the young of highly intelligent animal species spend more time in REM sleep than the young of less intelligent species. One day as I was explaining the light sleep/better brain correlation to a tired mother of a wakeful infant, she chuckled, “In that case, my baby is going to be very clever.”
Babies sleep healthier and safer. Not only do these immature sleep patterns help babies grow smarter, they help them grow healthier and sleep safer. Suppose your baby slept like an adult. Suppose baby slept so deeply that he couldn’t signal when he was hungry, cold, had a stuffy nose and was having difficulty breathing, or was just plain scared? Baby’s well-being would be threatened. Babies come wired to awaken so that they can let nearby caregivers know what they need to thrive and survive. What does this mean to parents? These arousals are thought to be protective arousals, and they are beneficial. Training babies to sleep too deeply, for too long, too young is not in the best interest of the baby’s development and well-being.
Sears’ Sleep Tip: Now that you understand infant sleep, when people ask, “How does your baby sleep?” you can answer, “Like a baby”.
There are sleep trainers who ignore these basic biological facts and insist that babies should be able to put themselves to sleep and sleep through the night. As you can see from the information in the previous pages, putting a baby down to sleep alone in a cot and leaving the baby to cry himself to sleep, and back to sleep when he awakens, is biologically and developmentally wrong. We are passionate about helping parents understand their babies’ basic sleep needs and giving them tools to cope until their babies reach sleep maturity, so we hope you’ll keep these biological facts in mind when making all decisions about your baby’s sleep.