Throughout our life our sleep needs change. Humans always desire sleep, but our sleep needs change as we get older. And it turns out our sleep needs change just before we’re born.
I’ve previously mentioned that healthy adults should get 7-9 hours of sleep. And this is the general recommendation, and it works for most people quite well.
But biologically, our sleep needs (or even our ability to sleep) change as we age, which can make sticking to the recommended 7-9 hours of sleep difficult. Additionally, when we sleep also changes throughout our life.
Understanding how sleep changes throughout our life is important so we know how to deal with these change. So today I’m giving an overview of how much sleep we need at each age.
Non-REM vs REM Sleep
Before we discuss how our sleep changes, I want to briefly review non-REM and REM sleep. This is crucial because the amount of time spent in these sleep states (particularly REM sleep) is mostly how our sleep changes throughout our life.
As an adult, we spend most of our time in non-REM sleep. This includes right when we fall asleep, deep sleep, and as we begin waking up. REM sleep is characterized by rapid eye movements and occurs as we transition out of deep sleep.
What’s important about REM sleep is that during this sleep state our brain is creating neural connections. This is also the sleep state where we typically dream. This is important to remember, because it will be vitally important as we explain how our sleep changes.
And we should also remember that REM sleep puts our voluntary muscles into a temporary paralysis. This obviously doesn’t affect any muscles required for survival, such as muscles controlling our breathing. But during REM sleep we are temporarily immobile, otherwise we could act out our dreams.
Sleep Before Birth
Before we’re even born we spend most of our time asleep. But this isn’t exactly what we would call sleep. Instead, we are in a sleep-like state that falls somewhere in between non-REM and REM sleep.
What’s interesting, though, is that this sleep-like state mostly resembles the brain activity we find during REM sleep.
Around 23 weeks of pregnancy, our brain has already created the centers that will control non-REM and REM sleep. Despite this, we still stay in a sleep-like state until the final trimester. What hasn’t developed yet is our ability to temporarily paralyze our muscles while in REM sleep. In fact, it’s these uncontrollable movements that create a baby kicking and “reacting” to external stimuli while in the womb.
Before the final trimester, we spend about 6 hours in non-REM sleep, 6 hours in REM sleep, and 12 hours in this sleep-like state. During the final trimester, we begin to experience some form of wakefulness. Two weeks before birth we increase our REM sleep to 9 hours. And one week before birth we increase our REM sleep to 12 hours.
We will never again experience this much REM sleep. But it shows how quickly the body is developing as it prepares for birth. Since REM sleep is associated with heightened brain activity and the creation of neural connections, it makes sense that an unborn child will need as much REM sleep as possible.
Sleep During Childhood
As soon as we’re born we begin sleeping in short, seemingly random bursts. And this is why new parents have a hard time maintaining a normal sleep schedule. The parents have a regular sleep schedule, but their infant does not.
This occurs because our circadian rhythm has not been developed yet. Even though our brain has all the mechanisms it needs to initiate sleep, the main force controlling our sleep is not yet in control.
Think of it this way: our circadian rhythm is largely controlled by the light-dark cycle of the world around us. We were not exposed to a light cycle inside the womb, so our brain doesn’t have the necessary data to compute a circadian rhythm. And this is why infants seem to have no regular sleep schedule.
At about three or four months old, an infant will begin showing signs of a regular sleep schedule. After one year we finally have a circadian rhythm that is in control. And by four years old, our circadian rhythm is finally the dominating factor controlling our sleep schedule. And by this point we have also transitioned from sleeping multiple times a day to sleeping twice a day (normal nighttime sleep, and an afternoon nap).
Finally, as we grow up our total sleep time decreases. What also decreases is our time spent in REM sleep. When we’re born we spend about half of our time in REM sleep. But by five years old we have a 70/30 non-REM to REM sleep split. So our total sleep time decreases, REM sleep decreases, but non-REM sleep increases.
Sleep During Adolescence
During our teenage years sleep is tricky. Of course it’s vitally important, but as we are well aware, our body is going through some big changes.
Before we were born our sleep was dominated by REM sleep. This was in order to create as many neural connections as possible. And this is why young children are so quick to learn new skills, because their brain has the connections available to them. But as we head toward adulthood, our brain decides it needs to be more efficient and less available for new connections.
The classic example is learning a foreign language. Young children are highly equipped to learn a new language. But as we get older, it becomes harder and harder to learn a language. This is why it is often argued that foreign language should be taught beginning in elementary school, because middle school students (around 13 years old) are already beginning to lose their language-learning connections.
So during adolescence our brain decides to fine tune itself and become more efficient by initiating more non-REM sleep, particularly deep sleep. And this is why it’s crucial for teenagers to sleep as much as needed, typically more than 9 hours. Their brain is in the process of maturing, and sleep is a critical element of maturation.
The teenage circadian rhythm is also noticeably later than an adults, and this is normal. Teenagers tend to stay up later than their parents, but this is a natural occurrence. Unfortunately, many parents don’t realize this, and this can become a source of tension between teens and parents.
Finally, sleep affects the teenage brain in a particular way. We know that sleep helps mature the brain into adulthood. But this process begins at the back of the brain and works its way to the front. So the last area to “mature” from sleep is the part of the brain responsible for decision making. This is one reason why teens often make irrational decisions. Part of their brain is an adult while part of their brain is still a child.
Sleep During Adulthood
As an adult we settle into a regular non-REM to REM sleep split of 80/20. And even as we age, this split will remain fairly consistent. What won’t remain consistent, though, is our time spent in deep sleep.
As we age we spend less time in deep sleep. This doesn’t me we don’t need deep sleep, because we certainly do. But it becomes harder for us to achieve deep sleep, and this is a natural occurrence. This can also occur due to certain medications that may interfere with sleep.
Another change that happens as we age is a change in our circadian rhythm. When we were teenagers, we naturally preferred to stay up late. But when we reach adulthood our sleep timing backs-up to what most adults would consider “normal.” And as we continue getting older, our sleep timing continues to shift earlier and earlier. This is why older adults tend to wake up earlier and go to sleep earlier (and why we have the early bird special at restaurants).
So this is why adults often complain of not getting enough sleep. We spend less time in deep sleep, our circadian rhythm is shifting, and this can lead to many interrupted nights of sleep. In fact, many adults experience a decrease in sleep efficiency, meaning it takes longer to fall asleep, and we wake up more frequently throughout the night.
We should also note that waking up becomes more difficult as we age. It is not so simple for us to quickly get out of bed, or get out of bed while still groggy. Older adults who get out of bed too quickly or while still groggy are more likely to suffer a fall, which can be serious.
The bottom line is that adults, especially older adults, need plenty of sleep. But our biology has made this task difficult.
This was an incredibly brief overview of how our sleep changes throughout our lifetime. Entire book chapters are dedicated to this topic, and this short post only scratches the surface.
But we should know and understand how sleep changes throughout our life. If we are prepared for these changes, and the changes in our children, we are more likely to combat or accept these changes.
Finally, this may serve as a good reminder to get the quality sleep you need early in life, because it only gets harder as we age.