How do our bodies tell the time?

Waking up each morning (or afternoon - we are students, after all) and throwing the curtains open, exposes your eyes to a vast array of colours, each composed of varying wavelengths in the visible light spectrum.


The light-detecting cells (photoreceptors) located in the retina of the eye detect these wavelengths of light and send signals to the visual cortex in the brain to form the picture you see - usually the bin lorry speeding towards your house after you have forgotten, yet again, to put the bins out. What you may not realise is that your eyes, and the photoreceptors within them, play a key role in stimulating the production of the hormones that help your body to work out the time of day. The human body clock serves to kick you into gear at the right times so that you can function throughout the day. Like many physiological processes, it causes changes in a 24-hour cycle known as the circadian rhythm.

Photoreceptors send visual information to specialised neurons known as intrinsically photosensitive RGCs (IpRGCs), which contain the signalling molecule melanopsin. This, when activated, communicates with the suprachiasmatic nucleus in the brain. This is a region of neurons in the hypothalamus known as the central body clock, which controls the circadian rhythm by telling glands to produce hormones such as melatonin and cortisol.

Melatonin is a sleep-promoting hormone that helps to reset the body clock. The daily timings and levels of melatonin release change from childhood to adulthood, giving lazy teenagers a reasonably valid excuse for later sleeping and waking times.

Cortisol is a steroid hormone released in response to stress and low blood glucose, serving to increase blood sugar levels and alertness. Levels typically peak in the mornings, acting as our body’s natural cup of coffee to help us get going.

Problems can arise when the body is unable to accurately tell the time, with jetlag being one of the simplest examples. When you’re away exploring different time zones, your body acclimatises and synchronises with the light cycle of the new environment (a process known as entrainment), so that the hormones melatonin and cortisol are being released at the right time of day. Step off the plane upon your return home and suddenly it’s all too difficult to fall asleep at 12am or stay awake in your 2pm lecture, until your body acclimatises again after a few days.

Some people have this problem permanently and suffer physiological and functional consequences from circadian rhythm sleep disorders, which cause their body clock cycle to shift. People who work night shifts or rotate between day and night can be affected, and blind people who are unable to perceive light at all. Capsules of melatonin taken orally may be used as a sleeping aid for sufferers of these problems.

But in the modern day, the sun is not the only source of light that can stimulate changes in the body clock. Many of us find ourselves glued to television or mobile screens late at night, completing essays or binge-watching Netflix shows, which can have a significant impact on our ability to sleep and therefore “reset” the body clock.

Backlights in LED screens used in technology emit lots of blue light, which is the optimal wavelength to stimulate melanopsin in the IpRGCs. This means that looking at your phone or laptop screen causes messages to be sent to the central body clock, incorrectly initiating a ‘daytime’ response. The release of melatonin is delayed, causing difficulty in falling asleep and getting up in the morning. This can heavily affect mood and learning capabilities. Blue light also causes the pupils to constrict, making it difficult to see in the dark as less Rod photoreceptors are stimulated. Astronomers use the longer-wavelength red light to avoid this when navigating their way around telescopes in the dark of night. It does not stimulate the IpRGCs, so that their eyes remain acclimatised to the dark conditions with heavily dilated pupils, and they’re not “woken up”.

This is a big problem for some unlikely members of society: elderly residents in care homes. Age comes hand-in-hand with a loss in bladder control (especially due to prostate problems in men), causing more frequent urges to urinate. Waking up in the middle of the night and turning on a bright white light in order to find their way to the toilet will expose their iPRGCs to lots of stimulating blue wavelengths. The health of residents is paramount, and an inability to sleep in regular patterns will severely impact this, with sleep deprivation linked heavily with depression.

Some “smart lighting” companies are now taking biological responses into account to help create a healthier environment. By varying the distributions of different wavelengths in the visible spectrum of light emitted throughout the day, the natural circadian rhythm is supported rather than disturbed.

So next time you find yourself unable to get to sleep late at night, you may cast your mind back to this article and suddenly regret cracking through those three Eastenders episodes before bed. In future, try to save them until the morning.

#GregorLawrence #Biology #Time #CircadianRhythm #Sleep

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