Chapter 20
A Good Night’s Sleep — Part 3
Moreover, the contribution of sleep to fighting infection isn’t just down to cytokines. Sound sleep also improves the action of immune T cells on fighting infection through a sticky tactic! ‘Killer’ T cells attack viruses such as influenza, HIV, herpes and Covid-19, by getting in direct contact with them, sticking to them and destroying them. The ‘sticky’ substance known as integrin is vital to allow the T cells to do this effectively but stress hormones such as adrenaline and noradrenaline block the stickiness of integrins. Because the levels of these stress hormones are low during sleep, there is a higher level of integrins in the body and they are stickier and thus better able to assist T cells in fighting off infection. Good sleepers are less likely to get winter colds and flus and are better able to fight infections should they occur. Chronic poor sleepers get more colds and flus and even have a poorer response to vaccinations. So, in summary, there are lots of ‘immune system’ reasons to work on better sleep. To fully understand why sleep matters for ageing we must focus briefly on circadian rhythms – our very own internal clock. Circadian rhythm has shot to the forefront of medical research in recent years. It is present in all living organisms and plays a major role in the changes that speed up ageing. Every cell has an internal clock that drives its circadian rhythm, which is synchronised to all other cells. Circadian rhythms are present to make the most of the cell’s abilities, to ensure that energy is not wasted and to give the cell and the body enough of a chance to clear all toxins which would otherwise build up, accelerating ageing and then cell death.
Chapter 5: A Good Night’s Sleep
A good example of a plant’s circadian rhythm is that of flowering desert plant Mirabilis multiflora, or the Colorado four o’clock plant. During the day, its flowers are tightly shut. After four o’clock, the blooms open for pollination and wilt the next day. For the petals to open, they require water to be transferred from the rest of the plant but given that it is a desert plant, water is scarce. The plant is pollinated by a nocturnal moth and so uses a ‘clock’ system to open its petals at four o’clock when it is cooler and the moth is circulating. This circadian clock ensures that the plant conserves water as much as possible during the hot day, as well as working efficiently to maximise pollination opportunities at night.
Chapter 5: A Good Night’s Sleep
Much like the Colorado four o’clock plant, our cells’ clocks work in synchrony – that is, at the same time and same rhythm, through a central control system located in the brain, called the suprachiasmic nucleus, or SCN. This is our master clock and it organises the clocks in every cell of our body for efficiency. It responds to the external cues of light, dark and food, and then orchestrates the clocks in all cells. The SCN helps us to wake up and be alert, tells us when it’s time to eat, ensures that our gut is awake and prepared to take in food and tells us when it’s time to go to sleep. The SCN is stimulated by light coming from the eye, which is why light and dark control circadian rhythm. All of the measures taken during a visit to your doctor – such as blood pressure, heart rate, temperature, blood levels of lipids, melatonin and cortisol – have circadian rhythms and therefore vary throughout the day. For example, blood pressure is lowest at night when we are asleep, peaks early morning and then settles to a daytime level which sometimes drops a little after a large meal or if we are resting. The SCN and your internal circadian rhythm is responsible for these blood pressure fluctuations. Ageing is closely linked to circadian rhythms and to maintaining a balance between sleep/wake and eating times. Circadian rhythms are controlled by the eye, the suprachiasmic nucleus (SCN).
The principal timekeeper gene that controls the clocks is the Bmal1 gene. Until 2020, Bmal1 was assumed to be the only time-keeper gene but researchers at the University of Pennsylvania found that skin and liver cells retain 24-hour circadian rhythm even after getting rid of this gene, indicating that while the Bmal1 gene heavily influences circadian rhythm, other genes are also involved. If we could manipulate these genes to be more efficient, it would slow down cell ageing.
Chapter 5: A Good Night’s Sleep
A key factor in the link between light–dark stimulation of circadian rhythm, ageing and sleep is melatonin. Melatonin is the hormone which regulates the sleep–wake cycle. Think of it as our body’s own sleeping tablet. It is primarily released by the pineal gland in the brain in response to darkness. Melatonin’s actions are not confined to sleep regulation – it also has antioxidant properties and beneficial effects on the immune system. In adults, it is mainly produced during the dark phase and the highest blood concentration occurs 4–5 hours after darkness. Light stimulus blocks the production of melatonin and as a consequence, during the daily light period its level is very low. Melatonin production decreases with age. With age, vision also declines and eye diseases such as cataracts become more common. These combine to reduce the intensity of the response of the eye to light and to lower melatonin further and reduce SCN stimulation. Early recognition and treatment of eye problems will help to minimise any negative effects of ageing on the SCN and melatonin and therefore on sleep. This is one reason why regular eye tests are recommended after age 40, which is when age-related eye problems may begin. With age, we experience a longer delay from sunset to the onset of melatonin rise and the melatonin peak. The relationship between age, declining melatonin production and increasing insomnia led to the ‘melatonin replacement’ hypothesis. Research shows that replacing the deficiency in this sleep-regulating hormone improves sleep. ‘Slow release’ melatonin tablets appear to be more efficient than faster acting melatonin. Melatonin at a dose of 2mg for up to 2 years has been approved for the short-term treatment of insomnia in people aged 55 years and over. This is a safe treatment with few if any side effects. Melatonin is also used for the short-term treatment of sleeping problems from jet lag or shift work. There is a deep-rooted connection between fires and human well-being. It’s lovely to sit around a ‘good fire’, which provides not only warmth but also comfort and relaxation, attributed in part to fires emitting yellow light. The ability to start and manage fire allowed the development of cooking and expansion of humans’ diets, and was a big part of our evolution as a species. Cooking also played a role in the expansion of our brains. The hearth formed a social focus, helping the development of language. Concrete evidence for the use of flints to start fires comes from as recently as 40,000 years ago but may have occurred as far back as 400,000 years ago. Therefore, until recent history, humans were predominantly exposed to, and their lives and evolution depended on, yellow light (wavelength 570–590nm) and blue light (wavelength 450-495nm) exposure was limited to a few hours in winter. Even the incandescent light bulb, widely used in the 20th century, produced relatively little blue light.
Chapter 5: A Good Night’s Sleep
However, over the past few decades, blue light has been used more and more in modern communication technologies, emitted from devices such as televisions, phones and computers. Blue light suppresses melatonin proportional to the light intensity and length of exposure, thereby causing sleep disorders and insomnia. The figure below shows how much blue light affects sleep. The longer the exposure before sleep, the shorter the duration of sleep. Email checking has the most striking effect, reducing duration of sleep by one hour if exposure goes from none to four hours. It is likely that the negative effects of blue light are exaggerated with age and therefore should be treated with even more caution. Glasses that block blue light in the hours before bedtime improve melatonin levels. Sleep duration and hours of screen use among 9,846 adolescents Although the 24-hour circadian clock is strictly regulated by the master clock in the SCN and complemented and assisted by melatonin, we do not all have the same cosy 24-hour relationship with our clock. Some of us are hard-wired to have our own preferred circadian pattern which does not exactly align to light/ dark circadian cycle. This matters because it helps to explain why some of us struggle with early rising when it’s light and with hitting the sack at darkness. This alignment to the circadian clock is called our ‘chronotype’, which is our very own personal circadian ‘hard wiring’ that governs circadian rhythm. Our chronotype describes our body’s natural timeline for the daily basic activities, such as eating and sleeping. The stereotypes of lark or owl represent chronotypes.