23rd January 2020
Ever wondered why babies need lots of sleep? Or why teenagers are practically nocturnal? Or why your retired dad falls asleep at around six? Sleep changes across your life as your body and brain develop and age. But it’s not just how much you sleep. It’s the pattern of your sleep, the way you sleep and what a lack of sleep can do to your body.
Before you’re born, you’re in a sleep-like state most of the time. When a baby kicks, it’s likely just dreaming, though probably not in the same way that we do. The fetus spends a lot of time in REM sleep (this is the stage in which we dream, where scientists think we consolidate memories) and the rest of the time in non-REM sleep or somewhere in-between.
It’s only in the third trimester that the fetus begins to wake up, but only for two to three hours a day. At the same time, REM sleep ramps up from around six hours to nine hours. About a week before a baby is born it goes up to 12. This is when the detail of the brain is being formed. Neural pathways are frantically being created all over the brain. At no point in the rest of a human’s life do we spend as much time in REM sleep!
Sleep helps babies’ brains develop, which is why they need up to 20 hours of slumber a day. But unlike adults, babies sleep for short, sporadic periods spread across the day. This is because their brains aren’t big enough to produce the sleep hormone, melatonin. Their pineal gland doesn’t produce enough of it consistently to keep babies asleep throughout the night, which is why new parents are often sleep deprived.
Brain development often occurs in REM sleep, so it makes sense that they spend at least twice as long as adults in this sleep stage. It will help them better adapt to the things that the big wide world will throw at them, it will help them to form memories and to learn new things.
When they are born, babies also don’t have a particular neurological barrier that prevents the body from “acting out” dreams and memories. This means that babies may twitch and kick out, sometimes even waking themselves up. As they reach six months and older they develop the barrier in order to properly inhibit any dramatic body movements.
Young children tend to sleep in one block at night, but with two or three naps in the day, totalling around 11-12 hours of sleep. This slowly declines until they’re six or seven years old.
In general, the sleep cycle is shorter in children, hitting around the 50-minute mark, as opposed to adult cycles, which are 90 minutes. They generally contain a lot of non-dreaming deep sleep.
One of the most important things for children and teens (and adults, but in particular for children) is consistent bedtimes. Variable bed times for children have been linked to lower school performance, bad behaviour and low self-esteem.
When it comes to teenagers, they typically need nine hours in a solid block, but at a later time of night. Their circadian rhythm skews slightly as they hit puberty, and their bodies and brains go through rapid development and change. The preferred time for going to sleep and getting up again are typically delayed until later on at night – it’s not laziness, it’s their 24hr body clock. That’s why school can be tricky – the early starts interrupt much needed sleep and force them to function on less rest.
Teens usually do return to an earlier bedtime once they emerge from puberty.
Non-REM sleep is really important for teenagers. During adolescence, the brain uses slow wave sleep to strengthen connections it uses often, and prunes back the ones it doesn’t, to make itself as efficient and effective as possible. This helps the brain mature and become “adult”. Interestingly, this pruning of little-used neural connections starts at the back of the brain. So, a teen can have adult capabilities when it comes to spatial awareness (located at the back of the brain) but critical thinking can still be in adolescence (located at the front).
The recommended amount of sleep for adults is somewhere between 7-9 hours a day and, in Britain, it’s generally taken at night.
The timings of your circadian rhythm and your lifestyle will have a huge impact on whether you feel well rested. If, for instance, you’re an owl (you wake up late and you go to bed late), then a job that starts in the early morning will disrupt your natural sleeping pattern, and you’ll struggle to get the amount of sleep that you need.
There are lots of other things that can affect your sleep as an adult too. Everything from sharing your bed with someone who snores and stressful life situations to pregnancy, having children and your physical and mental health.
REM sleep is generally stable throughout your adult life, but non-REM sleep starts to decline slowly in your late twenties.
As you get older, you may struggle to get the full 7-9 hours in one go like you’re used to, though you’ll still need the same amount. Typically, you’ll have fewer, shorter periods of deep, non-REM sleep, which translates as lighter, more fragmented sleep throughout the night. It also means that you’ll have less of the sleep that’s needed to boost your immune system, memory and other cognitive processes. As you reach your 80s, REM sleep begins to decline too.
Another thing that changes as you get older is your 24-hour body clock. The window for sleeping narrows, which is why older people generally wake up early and fall asleep in the early evening. Matthew Walker, in his book, “Why We Sleep” recommends some activities to help older people with their sleep:
“First, wear sunglasses during morning exercise outdoors. This will reduce the influence of morning light […] that would otherwise keep you on an early-to-rise schedule. Second, go back outside in the late afternoon for sunlight exposure, but this time do not wear sunglasses. […] Plentiful later-afternoon daylight will help delay the evening release of melatonin, helping push the timing of sleep to a later hour.”
Your quality of sleep may also be affected by chronic medical conditions, such as arthritis, sleep apnea or restless leg syndrome. Insomnia can be a side effect of medications used to treat these conditions, so getting to sleep and staying asleep can be trickier for older people!