What is sleep?
“Amazing breakthrough! Scientists have now discovered a new treatment that could reduce your risks of many diseases including heart diseases and diabetes, enhance your memory and make you live longer! It can also make you look more attractive and feel happier and less anxious.”. If you read this in a newspaper, would you be interested? This sounds very fictitious, but this is what Matthew Walker said about sleep in his international bestselling book ‘Why We Sleep’ (1). Simply getting sufficient and good quality sleep could bring you all those benefits.
Sleep is a common characteristic in all living organisms, and it’s controlled by the 24-hours circadian rhythm. It’s not clear why sleep has been preserved throughout evolution because, when you think about it carefully, when we are asleep, we can’t look after our children, we can’t gather food and we can’t socialise with others. When animals are sleeping, they are also vulnerable to predators. Sleep must then have some crucial benefits and through years of research, more and more benefits of sleep have now been discovered by scientists. We will begin with what is sleep…
There are 4 stages of sleep.
These 4 stages alternate throughout the night, and it takes approximately 90 minutes to complete a cycle. REM sleep duration increases with each cycle and this is the stage when most of your dreams occur (2).
Dreams commonly involve visual imagery, but they could involve thoughts and feelings as well. Some people may even physically move while they are dreaming. Let me share a personal experience. I once has a dream that I was reaching out for a test tube in a laboratory and in real life I also reached out my arm which caused me to accidentally push my cat off my be. He was not very pleased.
Why do we dream then? Researchers have proposed different theories about why we dream which include memory consolidation, processing emotions, and replaying a recent event. Some people think that it could just be a by-product of sleep and have no meaningful purpose (3).
Factors that affect sleep
The duration and quality of sleep can be affected by many internal and external factors. With increasing age, people are more likely to experience difficulties initiating and maintaining sleep which leads to shorter sleep duration and fragmented sleep. Researchers have found that sleep duration reduces by about 10 minutes every decade in women and about 8 minutes per decade in men (4). Older people tend to also experience lighter sleep. This is due to an increase in sleep stages 1 and 2 which are lighter stages of sleep, and a reduction in stage 3 and REM sleep (5).
It’s important to note that not everyone of the same age has a similar sleep pattern. So other factors must also play an important role in sleep.
Gender is one of the factors that has been explored by researchers. Women experience various unique hormonal and physical changes throughout life such as menstrual cycle, pregnancy, lactation, and menopause. These changes increase their risk of sleep disorders at different time points. For example, the risk of a sleep disorder called restless leg syndrome increases with the number of pregnancies, while insomnia is more common in postmenopausal women. Apart from hormonal changes, psychosocial factors may also play an important role, more midlife women have the role of caregivers which increases their stress level and in turn have adverse effects on their sleep (6).
Of course, there are many external factors that could have an impact on sleep. For example, jet lag when travelling abroad and shift work. In both cases, people experience misalignment in their circadian rhythm. If you travel from London and arrive in Sydney at 11pm local time in the summer, you may struggle to sleep as your body probably think that it’s just 2pm. Your circadian rhythm might need some time to adjust to this new time zone. Shift workers experience a similar situation, people on a night shift are forced to remain awake throughout the night and sleep during the day. If the working hours are changing frequently, their circadian rhythm could be misaligned leading to difficulties falling and remaining asleep.
Association between sleep and health
A large number of studies have been conducted by scientists to explore the impact of sleep on both physical and mental health. People who are sleep deprived have significantly higher risks of obesity, cardiovascular diseases, and type 2 diabetes. A review article analysed data from 45 studies with a total of 634,511 participants and found that “a reduction in one hour of sleep per day would be associated with a 0.35 kg/m2 increase in BMI” (7). There are many potential theories that could explain this association. This could be due to hormonal changes which promote hunger and more food intake. Or, by having less sleep, people are awake for longer and therefore have more chance to eat, which contributes to weight gain and obesity.
Obesity is a well-known risk factor for cardiovascular diseases and type 2 diabetes. A study reported that those who sleep less than 5 hours per night have a 151% higher risk of developing diabetes (8) and 82% higher risk of having cardiovascular diseases (9) than those who sleep 7-8 hours per night.
In addition to physical health, the impact of sleep on mental health has also been explored for many years. The UK Biobank collected extensive information on sleep and the health of the general public. Professor Simon Kyle, has published a study which showed worse performance on the reasoning test, basic reaction time test, numeric memory test, and visual memory test in those with frequent insomnia symptoms, as well as those who sleep less than 7 hours per night (10). Not only do your cognitive functions benefit from a good night of sleep, but your mood can also be dependent on it too. Studies have found a strong relationship between sleep duration and positive mood. Those with less sleep are 55% more likely to experience mood deficits (11).
In addition to your health, sleep can also have social impacts. The phrase “beauty sleep” has actually been backed up by science. A study invited participants to look at photographs of people who had an 8-hour sleep and the same people after sleep deprivation, without knowing when each photo was taken, sleep-deprived people were rated as tired looking and less attractive. This shows that humans are sensitive to sleep-related facial features (12). This may have social implications as this may impact the first impression that we leave people during special occasions such as interviews.
The importance of a healthy diet and physical activity has been promoted for a long time, but as shown by some of the research mentioned in this article people are starting to recognise the importance of sleep. As Matthew Walker said in his book, good sleep really is like a fictitious treatment that brings all sorts of benefits to your health and well-being.
Written by Gewei Zhu