Human sleep has become something of an optional extra for some of us. In a fast-paced modern world, ruled by mobile communication technology and “always-on” social media, it’s getting easier to ignore the traditional cues that it’s time to switch off and go to bed.
Another modern phenomenon is to cut down on sleep as a means to get more work done. Whether it is rising early to avoid the rush-hour or burning the midnight oil working at home, sleep duration is under threat.
All of which means we are going against nature and the rhythms that were embedded in humans centuries ago.
Chronobiology is the science of biological rhythms and cyclic processes in living organisms, including animals, plants and microbes.
Time of day
The level at which our body's function oscillates depends on the time of day. This 24-hour cycle is called the circadian rhythm and is synchronised by a circadian clock located in the brain.
The term circadian reflects the fact that each full period or cycle is not exactly equal to 24 hours. (It is now thought we have a circadian rhythm of about 24.5 hours.)
The clock alters hormone levels, digestion, body temperature, sleep and mood and is influenced by rhythmic cues from the outside world.
The cues are called zeitgebers (time givers) and they can produce both advances and delays in the body clock. The main zeitgebers are the light-dark cycle and the secretion of melatonin, although exercise also has some effect. In turn, this has led to a search for chronobiotics, substances that adjust the timing of internal biological rhythms.
Of the many classes of drugs claimed to possess such properties, melatonin and its agonists show the most promise when it comes to treating jet-lag, delayed or advanced sleep phase syndromes and shift-work sleep disorders. But research on the right dose and time of administration is ongoing.
Gene expression
Another active research area is the study of whether being awake, asleep or being in a particular phase of sleep affects gene expression.
From a medical perspective it would be useful to know if genes associated with particular diseases are either up-regulated or down-
regulated when a person is sleep-
deprived.
Then there is the whole area of chronotherapy – the name given to the timing of medication. In one of the earliest studies of cancer chronotherapy, Dr Bill Hrushesky, an oncologist at the University of South Carolina, described an experiment in which he switched the timing of chemotherapy in 31 women who had ovarian cancer.
He divided the women into two groups, with each receiving two standard cancer drugs, adriamycin and cisplatin.
One group received the adriamycin at 6am and the cisplatin at 6pm, while the daily schedule was
reversed for the second group.
The first group of women suffered approximately half the side effects, with less hair loss and less kidney damage.
But back to sleep and how it can affect our wellbeing. There is a well-proven correlation between sleep loss and impaired mental performance. Accumulated over a period of time, sleep loss shows a measurable effect on short-term memory and the ability to think flexibly.
Shift work is especially bad for sleep. The majority of the night-
working population never adjusts to a shift system, because to do so means developing an entirely different internal clock to the one with which the worker was born.
Evidence that sleep duration may affect the risk of cardiac disease comes from a number of studies showing a higher incidence of coronary heart disease in shift workers.
And just to show how complex the link between health and sleep can be, consider this: those lucky enough to enjoy nine hours’ sleep a night have 1.5 times the risk of developing heart disease than people who get by on seven hours. The land of Morpheus has many more secrets to give up, it seems.
mhouston@irishtimes.com
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