The role of sleep in maintaining brain function (2)
Last week we spoke about the role of sleep in health – both brain health as well as general health. It benefits weight control, blood sugar and blood pressure control, prevention of cancer and boosting of our immune system, amongst many other advantages.
Of course we know that lack of sleep also affects moods – makes both children and adults grumpy when they don’t get enough. Learning about the physiology of sleep is fascinating. Once again, as we delve deeper into the biochemistry and anatomy of this exquisitely designed process – far beyond the possibilities of random mutations – we stand awed and inspired. I will try to explain a very complex system simply.
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The sleep cycle is governed normally by the day/night, light and dark cycle. This produces what is called the circadian rhythm (circa – around and dia – day). Within the eye, on the retina (the back inner wall of the eye) are found retinal ganglion cells.
These light-sensitive nerve cells are able to detect light of different wavelengths and intensity, and relay this information to a control centre at the base of the brain (the hypothalamus) called the suprachiasmic nucleus.
A photopigment called melanopsin, is maximally sensitive to blue light which has a short wavelength. At the back of the brain is a small organ called the pineal gland, which produces a sleep-inducing hormone called melatonin. From the base of the brain are nerve connections to the adrenal glands near the kidneys, which are responsible for the production of cortisol. Blue light suppresses the production of melatonin, stimulating wakefulness. In contrast, soft (yellow) light tends to suppress the production of melatonin. This fact is important for sleep induction in the evenings.
Watching TV and working on screened devices expose our eyes to blue-wavelength light, which interferes with the production of melatonin, and inhibits onset of sleep. Melatonin has other functions – it causes blood vessels in the skin to dilate, resulting in warming of the skin and thus declining core body temperature. This process reaches its peak/trough around 4 hours after going to sleep. Thereafter blood levels of cortisol from the adrenal glands start to rise.
This switches off the production of melatonin, reducing vasodilation of blood vessels in the skin and increasing the core body temperature and raising blood pressure.
The normal timing of breakfast trains the adrenal glands when to start producing cortisol. Thus having regular patterns of sleep, meal times and waking times helps the system to become regulated and function optimally.
Exposure to morning bright light soon after wakening also sets the body clock. Vigorous exercise in the morning and gentle exercise in the late afternoon is also helpful in encouraging the normal circadian rhythm of hormonal cycles necessary for initiating sleep.
Of course the above explanation is quite simplified. We are only scratching the surface. Many other factors impact this finely tuned system – alcohol, dehydration, anxiety and stress, diet, night duty can all disrupt the normal circadian rhythm. We will talk about factors that can influence restorative sleep next week.
Stay well, and relish your sleep.
Dave Glass
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