Disruption of your natural circadian rhythms can cause health problems, so here are some tips for staying on schedule

Breakfast in the morning, work during the day, relaxation in the eveningfollowed by bedtime. The waking hours have a certain rhythm, and the24-hour cycle of the Earth’s rotation has resulted in a human sleepingpattern that largely takes place at night for about seven or eight hours ofcontinuous shut-eye.

Several studies have found that disturbing this ancient sleep-wake cycle canmake people more prone to illnesses such as diabetes, obesity, gastrointestinaldisorders and heart disease. Now, a study on nearly 300 pensioners at 24 GPpractices in England has found that the best time to have a flu jab is probably inthe morning, as this is the time when the immune systems of older people arebetter able to produce virus-fighting antibodies.

Scientists believe that up to 2,400 lives might be saved if all winter flu jabs weregiven to older patients before noon, as this is the time when their immunedefences are at their best. It is further evidence that timing can be critical – evenvital – for certain activities. So what else do we know about the “biological clock”that appears to control our circadian rhythm? Here are a few questions that can beanswered.

What is the biological clock and why do our bodies follow a circadian rhythm?
Scientists are now pretty sure that there are two kinds of clock at work in thebody. The central “master” clock of the suprachiasmatic nucleus, a collection ofabout 20,000 oscillating nerve cells in the hypothalamus of the brain, isintricately linked with daylight from the optic nerve of the eye. This internal clocksignals when it is time to be awake or asleep, and is linked with hormone levelsand body temperature, which correspondingly fluctuate over a circadian period of“about a day”.

However, more recently scientists have discovered that other tissues in the bodyhave their own peripheral clocks that are influenced by other environmentalfactors. Eating, for instance, can reset the peripheral clock of the gastrointestinaltract.

Does disturbing the circadian rhythm cause health problems?
There is clear evidence to suggest that it does. Shift workers who work at night,for instance, are known to be at higher risk of cardiovascular disease. One studyfound they were 40% more likely to suffer from this illness compared with non-shift workers.

One suggestion is that altering the body’s sleep-wake cycle causes an uncouplingof the central body clock of the suprachiasmatic nucleus and the peripheral clocksin the rest of the body, caused for instance by eating at inappropriate times overthe 24-hour cycle. And as the pensioners’ study showed, sleep can also influencethe activity levels of the immune defences.

So what is the best time to eat to stay healthy?
Studies on mice have clearly shown that allowing them to eat only when they arenaturally awake, which is at night because they are nocturnal, results in fewermetabolic defects. They are healthier and less obese than mice allowed to eat allday and night long. Similarly, a study by Frank Scheer at Harvard Medical Schoolon 420 dieters in Spain found that those who ate their main meal before 3pm lostmore weight than those who ate later. The message appears simple: make lunchyour biggest meal of the day.

When is the best time to sleep in order to be alert and creative?
This depends on the individual. Some people are night owls, some are morninglarks. Mimi Shirasu-Hiza, a sleep researcher at Columbia University in New York,says that even sleeping in one eight-hour stint might not be right for everyone.

In today’s 24/7 culture, is there a best time to engage with social networks?
Sleep refreshes the mind as well as the body and a study of 509m tweets sent overtwo years by 2.4 million Twitter users found that the most cheerful and upbeattweets are posted between 8am and 9am. The afternoon dip in concentrationmeans that between 3pm and 6pm people are more likely to retweet otherpeople’s tweets than create their own. The most emotional time for Twitter isbetween 10pm and 11pm when users were “alert and engaged, but also moreagitated”, researchers found.