Post No.: 0219
Age plays a key role in how much sleep is optimal for us and when we tend to sleep and wake up. Babies sleep for ~16 hours per day, with lots of randomly-timed naps throughout the day and night. They therefore have an ultradian rhythm (which definitely sounds like the lilt of a sci-fi alien species but is actually a biological rhythm with a repeating cycle that is much shorter than 24 hours long!) rather than a circadian rhythm (a biological rhythm with a repeating cycle that is around 24 hours long) to their sleep.
Very young children will then have only two then one daytime sleep periods per day, and they should be sleeping largely during the night now. Pre-teen children need ~10 hours of sleep per night and tend to find it hard to force themselves to stay awake hence they tend to conk out easily once bedtime arrives. But they do tend to sleep well and get up bright-eyed and bushy-tailed every morning. Woof!
Adolescents then experience anything up to a 3-hour phase delay in their body clocks, which means that they tend to naturally want to sleep later and get up later during puberty, and they also need ~9-10 hours of sleep per night too as their brains and bodies go through a growth spurt (sleep is crucial for growth and repair). So for those many adults who moan about ‘lazy teenagers’ sleeping in and sleeping late – you must surely remember how you were when you were teenagers?! (Along with all of the other traits associated with adolescents!) Many adolescents do end up being lethargic and (extra) moody and this is mainly because they are sleep deprived because the world wants to run at ‘9am to 5pm’ but they want to run a bit later. Moody, grumpy, selfish or antisocial people who claim to only need a few hours of sleep per night (e.g. some politicians) perhaps wouldn’t be so moody, grumpy, selfish or antisocial if they just got more sleep?! Read more about the health problems related to sleep deprivation in Post No.: 0123.
Most adults then need ~7-8 hours of sleep per night to be optimal, but there’ll be a moderate variance between different individuals. Then as people grow old, they will phase advance their sleep, meaning that they will gradually sleep earlier and get up earlier. Sleep will also tend to become more fragmented, so the elderly will wake up in the middle of the night more often on average than younger adults (which could be partially down to decreased bladder control due to ageing too). Men and women will reach this stage at various times on average too, hence couples may find a discrepancy between themselves and their partner’s sleep and wake times until they converge when they’re both over ~70 years old. One must also take into account one’s individual chronotype too (‘larks’, ‘owls’ and those inbetween).
Try to maintain ~7-8 hours of sleep per night even as you grow older (when our body clocks gradually make us want to sleep less per night time) – many symptoms of ageing seem to correlate with the symptoms of sleep deprivation (e.g. decreased insulin sensitivity, higher cardiac sympathetic activity, lower mood, alertness and vigilance, shorter short-term memories) so trying to maintain this amount of sleep might help one to delay these effects (bearing in mind that this is correlational data so the causes could be reversed or there could be another factor causing all of these symptoms). And as if it needs any more good things to be said about why we should all be doing it – exercise during the day also helps to improve or maintain our brain function and memory, and helps us to get a good night’s sleep too as long as it’s not done too close to bedtime!
The hormone melatonin (along with a whole host of different neurotransmitters such as dopamine, serotonin, noradrenaline/norepinephrine, histamine, orexin and acetylcholine) plays a role in how sleepy we feel. The less light our eyes receive (e.g. during the evening, unless we’re exposed to a lot of bright artificial – particularly blue spectrum – light), the more melatonin is produced and the sleepier we’ll feel so that we’ll in turn feel ready to go to bed at night. The hormone responds to the dark and light periods in the day and evolved in association with the amount of natural sunlight we naturally received at different times of the day (i.e. before artificial lighting was invented), thus regulates our ‘body clock’, which, for sleep and adults, has a ‘circadian rhythm’. As identified earlier, not all bodily systems run with a roughly 24-hour circadian rhythm e.g. the ‘ultradian rhythm’ of the sleep pattern of babies, or the 90-120 minute long cycles of the stages of sleep for humans; and the ‘infradian rhythm’ of the human menstrual cycle, or seasonal affective disorder (SAD, a mood disorder that affects people most commonly during winter), which have biological rhythms with a repeating cycle that is much longer than 24 hours long.)
Our internal body clock for sleep actually naturally runs slightly on the slow side, taking just over 24 hours per cycle (with a degree of variability between individuals) – it’s the amount of sunlight entering our eyes that is the most important thing in recalibrating our body clocks every single day. So if we don’t see enough sunlight every day (or possibly more specifically blue wavelength light during the day, for which the white light of sunlight includes and the daytime sky is usually blue) – our body clocks will gradually shift out of sync with a 24-hour cycle to make us want to sleep later and later each night, and therefore get up later and later each day (if we could). This happens a lot with people who live in extreme latitudes during certain times of the year, and people who work at night and so barely get to see the light of the day (which might also explain the groggy behaviour of zombies?! I’m sure they’re actually alright once they get enough sleep and you get to know them(!))
Note that natural sunlight is usually far brighter than any artificial light that we use at home so it can be difficult to substitute the sun with an artificial source of light for this purpose. It can be hard to tell objectively how bright something is compared to something else at different times because our eye pupils autonomously and without conscious control contract and dilate to regulate how much light we’re actually perceiving. But you can objectively check it if you have a light meter.
Also note that this body clock mechanism is not the only mechanism that affects our sleepiness e.g. the length of time we’ve been awake for since the last time we slept, the time(s) when we eat and are most active, and how physically or mentally active we’ve been and therefore how exhausted we are, all play a minor role in regulating our body clocks too. Levels of alertness also go through a daily cycle – we don’t only gradually feel tired at night but experience a dip in alertness around just after midday (which is the best time for a nap if you feel like you need one). So both the circadian rhythm and homeostasis (if you’ve been awake for a while then your body will signal the urge to sleep, and if you’ve been asleep for a while then your body will signal the urge to be awake, via your levels of the hormone melatonin and various neurotransmitters) are responsible for our daily wakefulness/tiredness cycles. The body has several body clocks that regulate other things such as hunger and digestion too, which can overlap with the body clocks involved with wakefulness/tiredness.
Feeling ‘overtired’ (the feeling that one cannot easily sleep once one has gone way beyond the time one normally sleeps) is biologically real – there is a ‘sleep gate’ or window of opportunity to drift to sleep easily when the circadian rhythm, melatonin levels and homeostatic drives meet optimally. Our eyes can also feel slightly painful to keep closed if we’ve been awake for a long time, especially if we’ve been staring at a screen, but this should ease within a few minutes.
We each individually have a natural chronotype too – ‘larks’ prefer to sleep and get up early and ‘owls’ prefer to sleep and get up late, but most people fall somewhere in the middle of this spectrum. One theory is that humans evolved so that some people are naturally ‘larks’ and some people are naturally ‘owls’ in order to stagger sleep times so that at least someone was awake to guard the family or community during the night. This had obvious survival advantages for the group as a whole for people living in the exposed environments of the past, but the modern world is primarily geared towards those with minor ‘lark’ or inbetween chronotypes, meaning that late or really early shift workers tend to suffer in this modern world. Your chronotype can therefore affect your health, work, social and academic life and lifestyle, and possibly your relationship with food too. One thing you could try if you don’t tend to feel tired when you want to be sleeping though, is getting up 15 minutes earlier than usual, and repeat with further 15-minute cuts to your sleep on following days until you do.
Woof! Once again, there’s more to say about this important subject that concerns about a third of our total lives, but let’s take a break, pace it out and come back to it some other time.