Some people leap out of bed each morning with the kind of bright-eyed enthusiasm that makes those around them reach groggily for knives; others only seem to come to life after noon and three strong coffees. Still others prefer the nightlife, which is not much good in triathlon.
How you greet the day is determined by a combination specific genetic factors, lifestyle and general attitude. But there is one common factor: we are all in thrall to our circadian rhythms.
They affect our body temperature, heart rate, blood pressure, blood glucose levels and other physiological patterns that help dictate endurance performance. Understanding them can help us perform better.
The rhythm method
Our circadian rhythms (the word circadian, derived from Latin, means 'about a day') are normally synchronised with the light/dark cycle, enabling us to sleep, work and perform more effectively at some times of the day than at others.
Most of us already have a preferred time of day for training but a little tweaking, taking account of our circadian rhythms, could mean more efficient training and better performance at races.
The temperatures rising...
Our body temperature has a major effect on endurance performance and our ability to work hard. Core body temperature drops to its lowest between 3am and 5am. It then begins to climb, dips a little in the afternoon - we've all felt that post-lunch slump - rises late in the afternoon and early evening, and then drops again later in the evening as we wind down and prepare for sleep.
Therefore we are predisposed to perform less efficiently early in the morning, at the time of lower body temperature. And we tend to perform better later in the day - swimming. biking and running place heavy demands on the cardiovascular, respiratory, muscular, nervous systems and energy metabolism, all of which are promoted by a rise in temperature.
It's probably no surprise to learn that a study of 25 experienced swimmers confirmed the effects of fluctuating body temperatures. The swimmers performed fastest over a series of six swim trials of 200m between 4pm and 6pm, and they were slowest between 4am and 6am. The fastest performances coincided with the swimmers' highest temperatures, and the slowest times with their lowest. The average time difference between fastest and slowest performances was considerable: 5.8 seconds.
Numerous other studies focusing on various indicators of optimal physical performance - blood glucose levels, rate of perceived exertion, body temperature, reaction times and even motivation - suggest we should be exercising and racing in the afternoon.
Blood glucose levels are at their lowest from 3-6am, further proof that, scientifically speaking, the morning is not the best time to go out for a swim, bike or run. Circadian rhythms of heart rate, respiration and oxygen consumption increase during normal waking hours, and our heart rate is about 20 per cent higher at noon than it is at midnight (even if you sleep at noon).
Triathletes are also more likely to be injured in the morning because muscle tissue is colder, joints and tendons are not as pliable and basal metabolic rate
is low. In various studies, hand-grip and leg-strength tests performed throughout the day show that our strength is lowest in the mornings, then peaks from 4-8pm.
However, these are general figures. Some people love to get out early in
the morning, and if that sounds like you, stick with your routine. You have adapted your training to suit your lifestyle. You are also, in chronobiological terms, among the 'larks' of the world; others are 'owls', while most people have a chronotype (circadian type) that falls between these two extremes.
Work with it
The scientific evidence is unambiguous but the fact is that most of us cannot train and race in the afternoons. Most people are busy during the day and many triathlons begin early in the morning. It's also important to remember that training at any time of day will benefit your fitness level and race performance.
If a race you've entered begins early in the morning, try to train in the morning, too. You'll be better prepared for that early start, in terms of getting your body moving and maintaining performance. But keep an eye on your taper; as you reduce training in the lead-up to a race, you may find you sleep a little later and eat your first meal later, too. Don't let your body become too used to the new, temporary, cycle.
Also, since circadian rhythms are linked to the day/night cycle, we tend to slow down in the darker months of the year and are less inclined to exercise.
If you know this is likely to happen you can take steps to adapt to the situation. In short, keep training.
To compensate for your lower core temperature early in the morning, especially in winter, when what passes for 'daylight' can seem a mockery of the word, add a few minutes to your warm-up to get your heart pumping before you begin your session.
Of course, at the other end of the day, the benefits of exercise are reduced if you leave it too late. Our circadian rhythms appear to be disrupted if we exercise too late in the evening. Exercise scientists believe that training too
close to bedtime leaves the sympathetic nervous system overstimulated for several hours, making it harder for us to get to sleep.
Exercising intensely for 20-30 minutes or more raises your body temperature and doing this close to the time you go to bed will delay your transition to deeper sleep because it will take you four to five hours to cool down. For this reason, it's recommended that you exercise no closer than three to four hours before bedtime.
However, vigorous exercise generally improves sleep quality. A comprehensive meta-analysis of sleep and exercise has shown that people who exercise fall asleep faster, and sleep longer and deeper than, those who don't exercise.
Whether you're a lark or an owl, or some other indifferent metaphorical bird that sits between the two, you probably do not need to radically alter your training schedule, but if you can fit in sessions when you're at your sharpest, you will feel the benefit.