Fetuses spend almost all of the time in a sleep-like state. It doesn’t yet have the part of the brain that causes muscle-atonia during sleep, thus explaining why babies in the womb kick and punch.
During the last 2 weeks of pregnancy, REM sleep in fetuses ramps up to 12 hours a day. This causes rapid synaptogenesis and building of neural pathways throughout the brain. In experiments with rat fetuses, disturbing REM sleep stalls construction of the cerebral cortex.
Alcohol impedes REM sleep in fetuses and babies, causing abnormal synaptogenesis. Once disrupted, a fetal brain may never fully regain normal function.
Because REM sleep is involved in emotional recognition and social interaction, disrupting REM sleep in utero might contribute to the autism spectrum.
While starting with very irregular sleep, babies eventually show more regular sleep patterns starting at 4 months, as their suprachiasmatic nucleus and circadian rhythm develop.
With age, total time sleeping decreases, and the fraction of REM sleep decreases. Now that the synaptogenesis of REM tapers off, NREM plays a larger role in brain refinement, pruning the associations that are most valuable unique to that child’s life.
Consider NREM to actually cause cognitive development - changes in deep NREM sleep precede cognitive milestones, and the last maturation is in the frontal lobe, dealing with rationality.
Caffeine exposure during childhood could reduce NREM sleep, delaying brain maturation and learning.
In puberty, teens develop a later biological clock than adults, preferring to stay up later and wake later. This isn’t just teens being rebellious - it’s in their biological nature. Asking teens to sleep at 10PM is like asking adults to sleep at 7PM.
The author theorizes that this is evolutionarily helpful for teens to gain independence from their parents (having time to be awake while their parents are sleeping). Moreover, teens do so collectively, and so they get private time to socialize.
Unfortunately, in the modern day, schools start at a very early hour (largely to match the circadian rhythms of adult parents). It’s far out of sync with the natural circadian rhythm of teens, so they tend to sleep late and wake up far earlier than they naturally would.
Considering all this, if you’re a parent, there’s no need to get frustrated at your teenage kid for seemingly being lazy and sleeping too much, when the environment is heavily geared against their biological tendencies.
Sleep quality starts deteriorating in the late 20s, with deep NREM sleep becoming impaired in length and power. In your late 40s, you’ll have lost 70% of deep sleep as a teenager; by 70, you’ll have lost 90% of deep sleep. Unfortunately, less NREM sleep worsens the ability to cement new memories in older people.
You often hear of the elderly sleeping little at night, so the natural conclusion is that the elderly just need less sleep. But this could be a myth. The elderly might be sleeping less because they’re unable to sleep for as long. This means they could still benefit from more sleep.
Seniors have three things going against them: 1) they sleep less, 2) they have less efficient sleep, and 3) they want to sleep earlier. This is caused by:
Exacerbating this are a few factors:
All this causes lower sleep efficiency - people in their 70s have 80% sleep efficiency, meaning they stay awake in bed for 1.5 hours when trying to sleep 8.
There are a few ways to get around this: