Changes in Sleep Across the Life Span
SLEEP BEFORE BIRTH
Through speech or song, expecting parents will often thrill at their ability to elicit
small kicks and movements from their in utero child. Though you should never
tell them this, the baby is most likely fast asleep. Prior to birth, a human infant will
spend almost all of its time in a sleep-like state, much of which resembles the
REM-sleep state. The sleeping fetus is therefore unaware of its parents’
performative machinations. Any co-occurring arm flicks and leg bops that the
mother feels from her baby are most likely to be the consequence of random
bursts of brain activity that typify REM sleep.
Adults do not—or at least should not—throw out similar nighttime kicks and
movements, since they are held back by the body-paralyzing mechanism of REM
sleep. But in utero, the immature fetus’s brain has yet to construct the REM-sleep
muscle-inhibiting system adults have in place. Other deep centers of the fetus
brain have, however, already been glued in place, including those that generate
sleep. Indeed, by the end of the second trimester of development (approximately
week 23 of pregnancy), the vast majority of the neural dials and switches required
to produce NREM and REM sleep have been sculpted out and wired up. As a
result of this mismatch, the fetus brain still generates formidable motor
commands during REM sleep, except there is no paralysis to hold them back.
Without restraint, those commands are freely translated into frenetic body
movements, felt by the mother as acrobatic kicks and featherweight punches.
At this stage of in utero development, most of the time is spent in sleep. The
twenty-four-hour period contains a mishmash of approximately six hours of
NREM sleep, six hours of REM sleep, and twelve hours of an intermediary sleep
state that we cannot confidently say is REM or NREM sleep, but certainly is not
full wakefulness. It is only when the fetus enters the final trimester that the
glimmers of real wakefulness emerge. Far less than you would probably imagine,
though—just two to three hours of each day are spent awake in the womb.
Even though total sleep time decreases in the last trimester, a paradoxical and
quite ballistic increase in REM-sleep time occurs. In the last two weeks of
pregnancy, the fetus will ramp up its consumption of REM sleep to almost nine
hours a day. In the last week before birth, REM-sleep amount hits a lifetime high
of twelve hours a day. With near insatiable appetite, the human fetus therefore
doubles its hunger for REM sleep just before entering the world. There will be no
other moment during the life of that individual—pre-natal, early post-natal,
adolescence, adulthood, or old age—when they will undergo such a dramatic
change in REM-sleep need, or feast so richly on the stuff.
Is the fetus actually dreaming when in REM sleep? Probably not in the way
most of us conceptualize dreams. But we do know that REM sleep is vital for
promoting brain maturation. The construction of a human being in the womb
occurs in distinct, interdependent stages, a little bit like building a house. You
cannot crown a house with a roof before there are supporting wall frames to rest
it on, and you cannot put up walls without a foundation to seat them in. The
brain, like the roof of a house, is one of the last items to be constructed during
development. And like a roof, there are sub-stages to that process—you need a
roof frame before you can start adding roof tiles, for instance.
Detailed creation of the brain and its component parts occurs at a rapid pace
during the second and third trimesters of human development—precisely the
time window when REM-sleep amounts skyrocket. This is no coincidence. REM
sleep acts as an electrical fertilizer during this critical phase of early life. Dazzling
bursts of electrical activity during REM sleep stimulate the lush growth of neural
pathways all over the developing brain, and then furnish each with a healthy
bouquet of connecting ends, or synaptic terminals. Think of REM sleep like an
Internet service provider that populates new neighborhoods of the brain with vast
networks of fiber-optic cables. Using these inaugural bolts of electricity, REM
sleep then activates their high-speed functioning.
This phase of development, which infuses the brain with masses of neural
connections, is called synaptogenesis, as it involves the creation of millions of
wiring links, or synapses, between neurons. By deliberate design, it is an
overenthusiastic first pass at setting up the mainframe of a brain. There is a grea

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