How Babies Sleep:
Life in the Womb Prepares for Sleep After Birth
by Rebecca Holman

Modern ultrasound techniques give researchers the opportunity to glimpse babies’ movements long before they are born. By the time you are eight to twelve weeks pregnant, your unborn baby can move in a variety of ways and has bursts of activity alternated with periods of rest.

During the periods of rest, your baby moves very little and is less sensitive to sounds and light from outside your womb. Some researchers think that the periods of activity can be related to periods of wakefulness and the periods of rest to periods of sleep after birth. However, sleep and wake states are defined according to neurological criteria and can only be truly identified when your baby’s brain is sufficiently mature.

As your pregnancy progresses, the cycles of activity and rest become longer and more regular. By the time you are about 20 to 28 weeks pregnant, a whole cycle of rest and activity lasts for between 40 and 60 minutes. Around this time, your baby’s arm, leg and eye movements become more synchronized with each other and with changes in their heart rate. This is considered to be a major milestone in their neurological development. When you are 30 weeks pregnant, your baby “rests” for over half of the time. As your due date approaches, the amount of time that the baby rests increases steadily.

In unborn babies, three sleep states have been described and can be identified on the basis of specific patterns of brain activity and changes in heart rate and eye movements. These states are known as “active,” “quiet” and “indeterminate” sleep. Active sleep can be seen as the precursor of REM sleep in older babies, children and adults. Quiet sleep can be seen as the precursor of non-REM sleep, although the stages in non-REM sleep only emerge weeks or even months after birth. Indeterminate sleep is when the criteria for none of the other states, including wakefulness, are met and is a result of a baby’s brain not yet being mature enough to organize all of its activities all of the time.

From when you are about 28 weeks pregnant, the periods that your baby spends awake and in active and quiet sleep states can be identified, although he or she still spends a lot of time in the indeterminate sleep state. Between 32 and 34 weeks, the amount of organization between a baby’s patterns of brain activity, changes in heart rate and eye movements increases remarkably. This means that, as you approach the end of your pregnancy, your baby spends gradually less time in the indeterminate sleep state and more in the quiet sleep state. Your baby also starts to display some of the characteristics of mature sleep-wake behavior. For instance, urination only occurs when the baby is awake and, if it is a boy, most of his penile erections occur when he is in the active sleep state.

During the last three months of your pregnancy, your baby’s circadian rhythms start to emerge. Circadian rhythms are variations in body processes, such as heart and breathing rates, temperature, blood pressure and the secretion of some hormones, which take place on a cycle lasting approximately 24 hours. These are controlled by a biological clock in your brain and help you to feel sleepy at night and awake during the day. Your baby’s heart rate is clearly synchronized with the periods that you are awake and asleep and the rhythms in your heart rate, body temperature and hormone production. At the end of your pregnancy, your baby has developed his or her own circadian rhythm controlled by his or her own biological clock. However, these are still supported by hormones from your body that pass through the placenta to your baby. After your baby is born, you can continue to support his or her circadian rhythms and biological clock by breastfeeding on cue, as these hormones also pass into your breast milk and are absorbed by your baby.

Influencing Baby’s Sleep

In your womb, your baby is not completely isolated from your environment. However, neither vibrations on or around your body or high levels of white noise seem to affect how he or she sleeps. Perhaps surprisingly, babies even appear to sleep during labor. Your unborn baby’s sleep can be influenced by recreational or prescription drugs that you use and by your state of mind. If you consume a lot of caffeine, your baby will probably spend more time “awake” than otherwise and if an opiate antidote were to be given to you, your baby would spend more time in active sleep and awake states than normal, particularly in the first hour after you had taken it.

If you take some types of antidepressants, your baby will spend more time in the active sleep state and be more easily startled after birth. Some pain relief medications that you may receive during labor can reduce the amount of time that your baby spends in quiet sleep. And the use of recreational drugs such as tobacco, alcohol, marijuana and more serious drugs like cocaine will not only harm your and your baby’s health but affect their sleep; smoking during pregnancy puts your child at an increased risk of sudden infant death syndrome.

If you are particularly anxious, it is likely that your baby will spend more time in the quiet sleep state and move less while sleeping, than if you are less worried.

Events during your pregnancy and labor can affect how your baby sleeps after birth and even into the preschool years. For instance, if your baby is your first or is born by Caesarean section, he or she may have more active sleep and less quiet sleep. Your mood and diet may also influence your child’s sleep after birth. If you are very anxious during your pregnancy, your baby may sleep for longer, but more restlessly, right after birth and have more sleep problems after the first few months. However, if you have higher levels of the fatty acid DHA in your blood, your baby may have more quiet sleep, spend more time awake and have fewer transitions between sleep states. Even if your pregnancy lasts more than 37 weeks, the amount of quiet and motionless sleep that your baby has after birth will increase as the length of your pregnancy increases. This indicates that his or her brain has reached a higher level of maturity.

The level of maturity that your baby’s brain reaches before birth is an important factor in how he or she will sleep after birth. You can support the maturation of your baby’s brain by eating a balanced and varied diet rich in omega-3 fatty acids and avoiding recreational drugs.

By preparing for labor well and obtaining information on natural methods of pain relief, you can minimize the probability that you will need medications during labor and even lessen your chances of requiring a Cesarean section. If you have good information and guidance about breastfeeding, you will have more chance in succeeding with nursing and hence in being able to support your baby’s circadian rhythms and biological clock with your hormones.

Preparing for Baby

Whatever you do to support your baby’s sleep patterns, it is unrealistic to expect your baby to sleep through the night in the first months or even years of life. It is normal for you and your baby to need night-time breastfeeds for months rather than weeks. You may need to breastfeed at night to reduce the possibility of your breasts becoming too full, milk ducts getting blocked, or even mastitis developing.

Your baby may need to breastfeed at night to ensure optimal growth, to prevent dehydration or low blood sugar levels, to maintain defenses against illnesses, to regulate bodily processes or to help him or her sleep. For this reason, it can be very helpful to prepare somewhere that you and your baby can rest and sleep together safely. Consider and remain open to a number of options, such as a bed for you and your baby to share, a separate cot or crib in your bedroom, a co-sleeper and a sleeping mat for you on the floor of your baby’s own room.

Above all, remember that your baby sleeps and wakes in a certain way because that is how babies are. Your baby does not wake up at night or demand closely spaced breastfeeds to annoy you, but because that is what he or she needs. As your baby grows and the brain matures, he or she will gradually start to sleep for longer periods.

Rebecca Holman is the mother of two small boys, Thomas and Daniel. She was born and grew up in the U.K. and has a degree in mathematics and statistics. She became interested in infant sleep after hearing a lactation consultant say that sleeping through the night was a matter of neurological development, not training. She has come to believe that the way breastfed children and their mothers sleep should be considered normal, and that mothers should not be pressured into “sleep training.” Read another article on sleep by this author.