Hours, minutes and seconds
How much of your day should you devote to exercise? According to the government, adults should spend 30 minutes performing moderate physical activity five days a week to stay healthy. But if that doesn't suit your lifestyle or body clock, there are other options. As little as 10 minutes of exercise a day has significant health benefits, from helping to whittle waistlines to controlling blood pressure. Indeed, a recent study in the International Journal of Cancer showed that just 10 minutes a week of exercise can reduce the risk of prostate cancer by 11%.
Experts agree that more time spent exercising is not necessarily better. The American College of Sports Medicine (ACSM) says that, often, people who plod away for lengthy periods on exercise equipment are clocking up "dead miles". In its guidelines, the ACSM says that 30 minutes of high-intensity exercise (that's at 80% of your maximum aerobic capacity, a level at which you would be puffing and sweating) is as good as an hour at a workload of 60%.
Meanwhile, a number of high-profile American fitness experts believe that strengthening sessions needn't drag on. Hollywood trainer Ken Hutchins says that by using heavier weights that are a high percentage of the maximum you can lift and slowing the pace of each exercise (instead of the usual four to five seconds it takes to lift and lower a weight, take 10 seconds each to raise and 10 to lower it), a weight session need last no longer than eight to 20 minutes.
"A heavy load on the muscles mean they gradually get fatigued to the point at which they can no longer lift," says Hutchins. "Within two to three minutes of exercising a muscle this slowly, it reaches a threshold. The body then gets a signal to make that muscle grow stronger." A study published in the Journal of Sports Medicine and Physical Fitness last year showed that short but intense regimes can produce 50% greater improvements in muscle tone than ordinary strength workouts.
Time of day
Professor Tom Reilly, an expert in chronobiology and exercise at Liverpool John Moores University, says that most research points to late afternoon and early evening as the best times to work out. In his own studies, Reilly found that when people were asked to perform the same workout at different times of the day (5am, 11am, 5pm and 11pm), they felt they were working hardest first thing. "Body temperature rises by a few degrees in the afternoon, warming the muscles and connective tissues, which contributes to an improvement in performance capabilities," he says. "Muscle strength is also better later in the day."
Top swimmers were shown to suffer a 10% drop in performance during morning training sessions. Researchers at California's San José University also showed that reaction times and hand-eye coordination, especially in sports such as tennis and football, improved after midday. Exercise in the morning and you are also more likely to catch a cold. A study in the British Journal of Sports Medicine showed that an early-morning swim left immune systems working less efficiently.
It is no coincidence, says Reilly, that most Olympic and world records are won in the late afternoon: "Most people experience a post-lunch dip in exercise performance between 2pm and 3pm, but generally their physical parameters improve as body temperature gradually rises throughout the afternoon. Stopping off at the gym on the way home from work is ideal as the body performs at its best between 4pm and 7pm." Exercising much later than that could be counter-productive. As your body winds down, you risk disturbing its natural biorhythms. "Levels of the stress hormone cortisol plummet and your brain starts to produce more of the sleep-inducing hormone melatonin," says Dr Tom Mackay, a consultant in sleep medicine at Edinburgh Royal Infirmary.
For early risers who prefer to exercise before work, sports scientists at Glasgow University revealed last year that morning gym workouts are better mood-boosters. The research, published in the journal Appetite, showed how women in an 8.15am aerobics class enjoyed a 50% boost to their feelings of wellbeing compared with 20% of those who worked out at 7.15pm. "In the morning the exercise seemed to feel much harder, but this means the emotional benefit is much greater," says Dr Siobhan Higgins, who led the study.
But all experts agree that working out at any time is better than not working out at all and that consistency is key to progress. American studies have shown that everyone benefits from working out at the same time each day - weight lifters who trained at the same time of day consistently gained more power than those who worked out at different times of the day.
Time of year
Last year, a study in the European Journal of Clinical Nutrition showed that although most people in the West gain weight in winter we are programmed to get fitter come spring.
Even the time of year that someone is born seems to affect their sporting performance: researchers have also shown that most of Britain's Olympic medal winners or world champions were born in October or November.