New evidence shows that keeping fit leads to better mental health and can help to stave off biological ageing by 10 years or more (Br J Sports Med, April 2008; doi:10.1136/bjsm.2008. 046243 & doi:10.1136/bjsm.2007.044800). But is there an optimal time of day to exercise? Should we drag ourselves up at the crack of dawn for a half-hour of jogging, or are we better off working out later in the day?
There's growing evidence that how you schedule your exercise can maxi-mize the benefits. The secret lies in our circadian rhythms, the daily cycles that regulate our physiological processes, such as body temperature, metabolism and blood pressure.
According to the American Coun-cil on Exercise (ACE), body tempera-ture is important for getting the most out of your workouts. It is lowest at one to three hours before waking up in the morning, and peaks in the late afternoon. Later in the day, muscles are warmer and more flexible, and reaction times are quicker, which means less injury and better performance. It's also likely that exercise will seem easier at this time of day, too (Student BMJ, 2003; 11: 219-62).
In fact, studies consistently show that physical performance is better in the afternoon and evening than in the morning. In male physical-educa-tion students, their strength, energy production and aerobic contribution increased from morning to afternoon along with the rise in body tempera-ture (Chronobiol Int, 2007; 24: 739-48). A similar trend was seen in swimmers, suggesting that swimming trials are best held in the evening, and not in the early morning (Br J Sports Med, 1983; 17: 122-7).
It also appears that we exercise harder and for longer later in the day than first thing, which means that we're likely to burn more calories. In high-intensity exercise tests with both men and women, the total work performed and time to exhaustion was nearly 10 per cent greater in the afternoon than in the morning (Can J Sport Sci, 1992; 17: 316-9; J Sports Med Phys Fitness, 1996; 36: 155-60). The greater tolerance for more intense exercise is also related to body temperature (Br J Sports Med, 1983; 17: 128-30), although increased levels of cortisol, which boost the amount of energy available for muscle activity, may also play a role (Am J Physiol Regul Integr Comp Physiol, 2003; 284: R714-24).
Lung function is better in the afternoon, too. Looking at nearly 5000 patients, researchers found that overall airway resistance peaked at noon, and dropped to a minimum at 4-5pm (Chest, 2004; 126: 744S). According to Dr Boris Medarov, lead author of the study, this suggests that exercising or engaging in other physical activity in the late afternoon is ideal as optimal breathing capacity contributes to optimal performance.
This means that, although we associate the end of the work day with feeling tired and less motivated to exercise, our bodies are, in fact, geared up for physical exertion at that time.
But what about early risers who swear by their morning routines? Everyone has his own rhythm and if beginning your day with exercise makes you feel good, then stick to it. According to the ACE, morning exercisers are also more successful
at making it a habit, and exercising regularly is more important than the time of day that you do it.
Also, morning exercise may help you to sleep better. A year-long study in postmenopausal women found that evening exercisers had more trouble falling asleep than morning exercisers (Sleep, 2003; 26: 830-6). A possible explanation is that morning exercise aligns the body clock, while evening exercise upsets it. However, a study in fit male cyclists found that vigorous exercise before bedtime had no effect on their sleep (Med Sci Sports Exerc, 1999; 31: 864-9).
In addition, if you exercise out-of-doors, air-pollution levels are typically at their lowest in the morning, so you're less likely to be exposed to hazardous pollutants.
Ultimately, though, it's important to find the time of day that works for you. The best time is the time that helps you to make exercise a regular, consistent part of your life.
Find your peak. To determine your own daily peak in body temperature, record your temperature every few hours for five consecutive days. It will usually fluctuate by about 1.5 degrees throughout the day. Try exercising during the period three hours before and after your highest temperature.
- Adapt. If you're training for an event, research shows that it's best to train consistently at the same time of day that you will be competing (Ergonomics, 1989; 32: 79-92).
- Use trial and error. If you haven't established a routine, try working out at different times of the day, then ask yourself which you enjoyed most and which made you feel the best afterwards.
- Warm-up properly. If you enjoy early workouts, make sure your warm-up emphasizes stretching to ensure that your body is ready for action.