There is little meaning any more to dividing the day into morning, afternoon, evening and night-time. Our shiny new 24-hour society requires us to work when we have to, sleep when we can, eat on the hoof and shop in the early hours. But by breaking all the time barriers, we are driving our bodies beyond the limits of our biology. No wonder most of us complain about always feeling tired: we are fighting our natural daily rhythms — and losing.
Humans have destroyed many links with the natural world. Our food comes prepacked, our drink bottled and we take pills instead of chewing leaves. Electricity turns our nights into days, and central heating, our winter into spring. But if we were to go deep into a dim cave without a watch, the modern mask would fall away after a few days out of the sunlight and we would naturally revert to ancient patterns. Deprived of time cues, our daily rhythms of eating, wakefulness, thinking and sleeping slowly drift out of alignment with the outside world and go back to being determined by our body clock. Our bodies are ruled by these internal timers, which produce a daily cycle of peaks and troughs in hormones, performance, alertness and behaviour.
We think best, for example, around midday, so this is the ideal time to challenge your boss or to do any tough problem-solving. It is not best spent going out for an hour’s lunch break — unless, of course, you’re going on a power lunch, which, despite the 1980s Working Girl image, probably has some basis in age-old biology.
If you’ve made it as far as midday, you can breathe a sigh of relief, since you have survived the most dangerous part of the day — most deaths occur in the early hours. The majority of heart attacks, for example, happen at about 6am, as blood pressure begins to rise. This rise dislodges plaques that have formed during the night, which, if sent flying into blood vessels in the heart or brain, result in a heart attack or embolism. A rise in blood pressure is also thought to be the reason why migraines tend to peak in the morning, too, at about 8am.
Most of the time, we don’t pay any attention to these rhythms, but even our moods and emotions, including our most intimate moments, are subject to our internal clock. The favoured time for having sex, for instance, is 10pm, when we are at our most sensitive to touch. Strenuous sports, or stretching exercises such as yoga and Pilates, are best done in the evening, when we are at our strongest and most flexible. By about 8pm, our body temperature — which rises throughout the day — peaks, so our muscles are warmer. This explains why an Olympic athlete can swim 100 metres 2.7 seconds faster at 8pm than at 6am, the time when training sessions usually take place.
These rhythms also make nonsense of our modern diet. After 5pm, it takes more food to make us feel full, so we tend to eat more, particularly as our taste buds are at their most receptive later in the day. However, this late-eating habit is not particularly good for our health. For example, our ability to process triglycerides (a form of fat carried through the bloodstream, high levels of which can contribute to heart disease) is impaired after midnight. It makes sense, therefore, to use this information and provide night-shift workers, who have a higher incidence of coronary heart disease, with easy-to-digest high-protein, low-fat meals.
The rhythms apply not only to our daily clock. In a throwback to the way our primitive ancestors coped with seasonal fluctuations, we tend to eat more fat in summer, carbohydrates in the autumn and proteins in the winter. In spring and summer, the body uses carbs as its main source of fuel and stores fat, which serves as the primary fuel source in autumn and winter. This was fine when food supplies regularly went from abundance to scarcity, but it plays havoc with our weight in an era when food is plentiful all year round.
Most of us have a similar time signature that charts the best part of the day to perform various tasks. However, studies have shown that about 10% of us are genuine early-morning people, or larks. Another 10% or so are owls and are more switched on in the evening. If you are a genuine owl, doing a night shift is fine. If you’re not, it’s pretty horrific, because you can never fully adapt while your biological clock is telling you to rest. You can override its influence to a certain degree, but you can never expect a peak performance. Of course, the added problem for night-shift work is that you have to fight your natural rhythm by sleeping during the day, when your clock is telling you that you should be awake. The result is huge sleep loss, which, in turn, exaggerates the problems of a badly adapted physiology on the night shift — and so the damaging cycle goes on. Clinical trials have found that our ability to perform numerical tasks between 4am and 6am is comparable to those carried out by people who are drunk.
Ideally, we should be rising at dawn and going to sleep at dusk. But try explaining that to your average teenager on a Monday morning. Only 10% of teenagers (particularly boys) have a valid excuse for wanting to remain under the duvet: it is the response of their biological clock to the changes puberty brings that is to blame. However, the unaffected 90% of teenagers can get out of bed, and those who claim they cannot are malingering. But they do grow out of it.
Chronobiology, the scientific study of these daily rhythms, is still in its infancy, but it is already beginning to have profound effects in medicine. Instead of the present pattern of one tablet given three times a day, medicines are increasingly being delivered in carefully calibrated doses at specific times designed to maximise their effect. For example, medication for rheumatoid arthritis is given in the morning, when pain is most acute, while drugs for osteoarthritis are given in the early evening, when symptoms peak. The diagnosis of conditions such as breast cancer is also improving. Breast tumours tend to grow in springtime, probably due to seasonal changes in our immune system, and are therefore more easily detected then.
This is just the beginning. One day in the future, our time signatures will be as important in fighting disease as knowing our blood type. Each of us will have to learn to become time wise and not time foolish.
Rhythms of Life by Russell Foster and Leon Kreitzman is published by Profile at £20. To order your copy at £15 + £2.25 p&p, call The Sunday Times Books First on 0870 165 8585 or visit the website: www.timesonline.co.uk/booksfirstbuy
