Ever felt as if you are run by clockwork? That's because to some extent you are. A biological clock is ticking in every cell of your body.
These clocks respond to signals from a patch of your brain which keeps them synchronised with each other, and with the time of day. It's called the suprachiasmatic nucleus.
Containing 20,000 cells and no bigger than a grain of rice, it is the biological equivalent of the Greenwich Meridian; it's the reference point used by the billions of other cellular clocks in your body to remain accurate and maintain the circadian rhythm of your body.
The normal working of the brain, immunity, and bodily functions such as heart rate or digestion, are all controlled by this rhythm — disrupt it, and it can damage your health and recovery from illness.
Yet these rhythms can be disrupted by many things: bright light at the wrong time of day; chaotic eating or exercise patterns; certain medications.
Even the microbes in our gut have circadian rhythms, which can be disrupted by antibiotics or eating at unusual times. Studies in mice suggest that when this happens, it may further impair the function and rhythm of our organs.
New understanding of how the clocks in our bodies behave is being used to explore how we can recover from illness and make drug treatments work better — with fewer side-effects
Now, new understanding of how the clocks in our bodies behave is being used to explore how we can recover from illness and make drug treatments work better — with fewer side-effects.
The body clock is so intertwined in the healing process that even the time of day you experience an injury, or an event such as a heart attack, may have an impact on how well and how quickly you recover.
For example, we know that the cardiovascular system has a strong circadian rhythm: blood pressure is lowest when we're sleeping, but it rises sharply upon waking up; our platelets — small fragments in the blood that help it to form clots — are stickier during the day; and the levels of 'fight-or-flight' hormones such as adrenaline, which constrict our blood vessels and make the heart beat faster, are also higher in the daytime.
These circadian variations affect the likelihood of a heart attack: statistically, you're more likely to have one between 6am and midday than at any other time.
Timing may also affect our ability to recover from heart injury. Studies in mice have revealed differences in the type and number of immune cells that infiltrate injured heart tissue, depending on what time of day the injury occurs.
This affects how much scar tissue forms and how well the heart subsequently pumps blood — which ultimately impacts survival.
Human studies have also suggested that patients' survival prospects are improved if they have heart surgery in the afternoon rather than in the morning.
WHY PAIN IS WORSE AT CERTAIN TIMES OF DAY
It's not only the cardiovascular system that shows this variation. A recent study found that skin cells called fibroblasts, which play a key role in wound healing, work more efficiently during the day than at night because of fluctuating levels of proteins, which direct the cells towards injured regions.
Skin wounds that were inflicted during the night (when mice are awake and active) healed faster than those inflicted during the daytime.
When the same researchers analysed data from the International Burn Injury Database, they found that people who suffer burns during the night take approximately 11 days longer to heal than those injured during the day.
There are other examples of circadian variations in our physiology: viruses find it easier to replicate and spread between cells at night; allergic reactions are strongest between 10pm and midnight; while joint pain and stiffness are worse in the early morning.
The seasonal flu vaccine was recently discovered to generate four times as many protective antibodies if taken between 9am and 11am, compared with when taken six hours later. File image used
Disruption of these rhythms — which commonly happens in a hospital environment — could impede people's recovery.
UK guidelines for intensive care units recommend natural daylight in every patient's room, as well as artificial lights which can be turned up or down.
You don't need to be regularly flying across time zones for your internal clocks to become scrambled. Irregular bedtimes and too much exposure to light at night can also play havoc with your inner rhythms.
As well as going to bed and getting up at similar times each day, one of the cardinal rules for all-round good health is sticking to similar mealtimes.
'If your food is arriving at a regular time of day, you want your metabolic clocks synchronised to when you're going to eat, so they can process it efficiently,' says Jonathan Johnston, a reader in chronobiology at the University of Surrey.
Gerda Pot, a visiting lecturer in nutritional sciences at King's College London, investigating how irregularity in people's energy intake affects their long-term health, studied more than 5,000 people and discovered that those who had a more irregular meal routine had a higher risk of developing metabolic syndrome — a cluster of conditions including high blood pressure, raised blood sugar levels, excess fat around the waist and abnormal cholesterol levels, which together increase the risk of cardiovascular disease and type 2 diabetes — even though they consumed fewer calories overall.
When mice were fed in the daytime, when they would usually be asleep, scientists found that they sustained more skin damage in response to UV light compared with those fed at night. As well as eating, exercise should also be taken at similar times every day as going for a run or to the gym at unexpected times — such as just before bed — may disrupt your body clock.
Take every opportunity to get outside during the daytime, as light keeps the master clock in the brain synchronised to the external time of day. Exposure to morning light is especially beneficial as the master clock is most responsive to it then, and it can help make us feel more alert.
And turn lights down at night: use dimmer switches or even switch off some lights. Bright light boosts alertness and pushes the timing of our internal clocks later, making it harder to sleep.
However, even in hospitals that follow this guidance, bedside illuminance during the daytime is well below the levels found even shortly before sunset outdoors.
More generally, modern hospital buildings are often characterised by small windows and dim indoor lighting that remains switched on day and night.
Yet exposure to light at night and an absence of bright light during the day can cause circadian disruption, sending the rhythms in our cells out of sync. This doesn't only disrupt sleep, which is itself essential for recovery; it has a direct impact on healing too.
For example, a large study of Canadian patients recovering from heart attacks found that the mortality rate among those recuperating in brighter rooms was 7 per cent, compared to 12 per cent among patients in gloomier rooms.
Compounding the problem, certain drugs, including morphine, can alter the timing of circadian clocks, while patients' sleep may be further disrupted by pain, worry or noise.
No surprise, then, that