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Thursday 2 April 2020

How much do unborn babies sleep? (Things We Don’t Know about Pregnancy Series #16)

We can’t measure the brain activity of a human foetus – not whilst they’re inside their mother. But we’re really interested. What happens to a baby’s brain as it’s developing, and what does this tell us about our own and the developing process of sleep?

EEG (electroencephalogram) reading via Wikipedia Commons.
Researchers into brain activity have performed EEG (electroencephogram) exams on premature babies, and monitored eye movement in the womb to learn about sleep cycles, although big errors with these kinds of measurements are common. They have, however, detected REM (rapid eye movement) sleep from around 7 months, when the brain cycles in and out of restful and REM (rapid eye movement) sleep every 20 to 40 minutes. From 7 months onwards, foetuses mostly sleep (90-95% of the time at 32 weeks, 85-90% by 37 weeks). After it develops, REM sleep increases and increases, reaching a lifetime high of 12 hours a day just before the baby is born.

Very little is known about foetus sleep before this. For example, we don’t know if sleep and sleep cycles suddenly or more gradually develop with the foetal brain. New research into lambs has shown that foetuses enter a dreaming-like brain state weeks before REM sleep starts[1]. As well as learning more about sleep, this study could help us figure out how the brain develops and when it is most at risk.

Equally, neurons in the brain develop much earlier, with those responsible for sleep present long before REM sleep is seen.

When an unborn baby first starts to move, its motions are involuntary, but they soon become voluntary – at around 16 weeks. Gradually, it learns to control its own limbs and respond to stimuli around it. But… it’s mostly sleeping, right? So how is it responding?

This means many of the motions are instinctive responses to outside pressures – and possibly, although we don’t know, dreams (it’s believed that adult-like dreams develop around age 5). Unborn babies don’t have their limbs paralysed when they sleep to stop them from acting out their dreams. This is something that happens to adults, and leads to the phenomenon of sleep paralysis, which you can (read more about here).

Alas, there’s no evidence the movements and routine of unborn babies codes for the movements and routines of born ones, although lots of anecdotal information says there might be.

Baby sleeping. Public Domain via TawnyNina (Pixabay)
Opinion is divided when it comes to whether foetuses sleep more at night or day, and move more at night or day, although lying down can trigger movements when the baby gains space and stretches out. Even after birth, babies sleep randomly until around 3 months, when their brains start producing their own melatonin – before this, they rely upon the mother’s body clock, and melatonin passed in breastmilk. But does it even work to make babies sleep preferentially at night? Many babies are soothed by their mother walking, and this persists after birth, with motion such as rocking sending them to sleep, whilst stillness encourages them to wake up.

Another area of research is the relatively un-navigated territory the relationship between foetal health and maternal sleep. Some studies think that the mother can influence her child’s health by sleeping well when she’s expecting, but the cause-effect relationship between impaired sleep and infant health is a difficult web to untangle!

There are many unknowns when it comes to pregnancy, and over the next few months, I’ll be exploring more of them with you. Look out for my next blog post, which will be about birth trauma.

To read our full article on the things we don't know about pregnancy, check out our site. This article will be updated as we add posts across the coming months.

why don't all references have links?

[1] Schwab et al. Nonlinear analysis and modeling of cortical activation and deactivation patterns in the immature fetal electrocorticogram. Chaos An Interdisciplinary Journal of Nonlinear Science, 2009; 19 (1): 015111 DOI: 10.1063/1.3100546.

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