The science of sleep - Part 2 - And So To Bed

Sleep and ageing


Sleep changes as we age. Babies and young children have far more deep slow wave sleep (stages 3 &4) than adults, and this decline continues across our lifetime with some elderly people showing no deep sleep at all. Also, newborn babies spend almost half of their time asleep in REM sleep, but by the time they are two years old it has gone down to around the level of an adult (25 per cent). As we age, we wake more frequently during the night and often go to the toilet in the night. This is usually nothing to worry about, unless you struggle to get back to sleep.


Spring cleaning your brain

During sleep your brain gets flushed with cerebrospinal fluid to “spring clean” it. It is suggested that when we do not sleep enough our brains clog up with waste products that have not been removed (including amyloid plaques) which could be linked to diseases such as Alzheimers. 


Drives for sleep

There are two main factors that control when we sleep: our circadian rhythm and our homeostatic sleep drive.


Our circadian rhythm

Our circadian rhythm is our own internal 24 hour clock. This is the clock inside us which tells our bodies when it is time to sleep, when it is time to wake, when it is time to eat, when to be alert and or drowsy and regulates our hormones and temperature. Our body temperature increases during the daytime and falls at night. Most people don’t even notice their own circadian rhythm until they go abroad on holiday to a different time zone, when it goes wrong… For example. if your body is used to being on “UK time” but you are now in America, then your circadian rhythm will tell you that its time for bed and make you very sleepy, when the local clocks tell you it it is the afternoon and time to explore the city!


Our circadian rhythm is mainly regulated by sunlight. Just like flowers that know when to open their petals to the sunshine, our bodies use the sun to know when it is daytime. This means that you can boost your circadian rhythm by making sure that you get outside to get some natural sunlight, ideally between the morning and midday. This is a great way to fight jetlag. However, some electronic devices can interfere with this process. Devices such as phones and tablets produce “blue light” which is the same frequency light as our brain uses to identify day and night, so when we use them in the evening we are telling our brains that it is the middle of the day - and that we should not be go to sleep anytime soon. In fact, our circadian rhythms make us naturally produce a hormone called melatonin in the evening to tell our bodies to start going to sleep, but the blue light from tablets and phones suppresses this natural production of melatonin - stopping us being able to go to sleep easily.


The homeostatic sleep drive

The longer we are awake, the greater our “drive” for sleep. This is known as our homeostatic sleep drive. If we have been awake for a very long time (such as staying up very late into the early hours or the morning) then this drive for sleep will keep increasing. In contrast, when we take a nap we reduce our homeostatic sleep drive. This means that an afternoon nap will make you feel refreshed, but also reduce your drive for sleep that night - that is why you might struggle to fall asleep in the evening if you took a nap during the day.


How does caffeine affect sleep?

Caffeine stops our brains from being able to recognise our homeostatic sleep drive. Even if we are very tired as we have not slept for a long time, when we drink caffeinated drinks or chocolate the caffeine stops our brains from being able to tell that we need sleep. Most importantly, caffeine has a 5-7 hour half life, meaning that 5-7 hours after a cup of coffee, half of the caffeine will still be in your system. It is best to limit caffeine to the morning, to give you the best chance of getting to sleep at night.

Dr Lindsay Browning is a Chartered Psychologist, Neuroscientist and sleep expert at Trouble Sleeping, author of the self-help sleep book Navigating Sleeplessness and can be found on all social media @DrBrowningSleep.

Posted by Dr Lindsay Browning
8th May 2019

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