What time is it inside your body? The answer may determine the effectiveness of your training - and your performance
'Time of Day May Influence Athletic Performance', teased recent headlines in major newspapers around the world. Athletically minded readers, skimming the stories to discover their optimal time to play squash, cricket, or soccer, found a summary of a report from the 83rd Annual Meeting of the American Endocrine Society in Denver, Colorado, which indicated that exercising in the evening might be best since it seemed to boost levels of key hormones involved in energy metabolism.
As part of their research, Dr Orfeu Buxton and his colleagues at the University of Chicago studied 40 apparently healthy men aged 20-30, dividing them into five different groups. A control group did no exercise, while members of the other four groups worked out at different times of the day, namely morning, afternoon, evening and night. Their workouts were identical, comprising one hour of vigorous activity on a stair-stepper machine.
While not exercising, all the subjects rested in bed, being fed intravenously with a glucose solution in a bid to maintain constant blood-glucose levels. Before and after exercise, the researchers measured glucose levels as well as blood levels of four key hormones: cortisol, thyrotropin, growth hormone, and melatonin.
When the results for the four groups were compared, it turned out that hormonal responses to exercise were strongly influenced by when the exercise took place. For example, night-time training produced a dramatic drop in post-exercise blood-glucose levels, while morning, afternoon, or evening exertions had minimal effects on glucose metabolism. Night and evening exercise produced large increases in cortisol and thyrotropin levels in comparison with morning and afternoon workouts, while growth hormone responses seemed unaffected by the timing of exercise.
Since thyrotropin is a thyroid hormone, which boosts metabolism, and cortisol tends to increase the amount of energy available for muscular activity, it is only logical that Buxton would ponder, as he did, 'whether regular evening exercise is better than regular morning exercise'. (He was less keen on the idea of nocturnal exercise, since it might involve sleep deprivation!) In the light of Buxton's results, one might expect evening activity to be more prolonged and higher in intensity than training at other times of the day, simply because of hormone-mediated increases in fuel availability. As a result, evening exertions might be especially fitness enhancing. In addition, it might lead to greater weight loss, partly because of the increased intensity and duration of the exercise but also because of thyrotropin's stimulating effect on metabolism.
All sportspeople's bodies are governed by their inner clock
'We did not expect the effects of exercise to vary to such a large extent depending on the time of day,' Buxton told Reuters Health. But, in fact, he should not have been surprised by his findings, since all sportspeople's bodies operate according to the dictates of an inner clock.
For example:
1. Your spine is a better shock absorber in the morning than the evening, while the strength of your back muscles increases as the day progresses;
2. Your blood is stickiest - and clots most readily - at around 8-9am, becoming less viscous as the day progresses: one reason why most heart attacks occur in the morning;
3. Your joints are stiffest early in the morning and most flexible late in the afternoon, while your muscles are strongest at about 5pm, weakest at 8am and 8pm and most supple at about 6pm;
4. Your airways are open to the fullest extent at 4pm, and your body temperature peaks at approximately 5pm, about the same time as your white blood cell production reaches a peak;
5. Your maximal aerobic capacity (VO2max) tends to be greater in the afternoon than the morning.
What these strange facts tell you is that your body is full of different rhythms - regular alternations of activity and inactivity. Most of these rhythms have a natural, repeating 'period' (with one high point and one low point of activity) of about 24 hours, so they are known as 'circadian' rhythms (from the Latin circa diem, which means 'around one day'). Some patterns, such as menstrual cycling in women, are called 'circamensal rhythms' because they have a period of about one month, while other are 'circannual', including the southward migration of geese (or your frantic search for your golf clubs in the basement each spring!)
Note that the circadian rhythms mentioned above seem to boost your capacity for exercise in the afternoon, when muscle strength, respiratory capacity, aerobic capacity, and immune function peak, and body temperature reach a level, which increases muscular efficiency. Indeed, athletes often report that workouts feel easier in the afternoon, and studies have shown that cyclists take longer to become exhausted in the afternoon than the morning. Most world records have been set in the afternoon or evening, although it is fair to point out that, in fact, few competitions are scheduled for the morning.
Should you always plan to train in the afternoon?
In practical terms, what should you make of all this information about circadian patterns? If afternoon exertions are of higher quality than morning efforts, shouldn't you always try to train in the afternoon? To test out this theory, researchers at the University of Georgia asked one group of runners and cyclists to train only in the afternoon and another group to work out exclusively in the morning. Each group trained in exactly the same way, with three interval workouts on an exercise bike per week (consisting of five five-minute intervals at about 95% of max heart rate, with two-minute recoveries) and two 30-40-minute sessions of intense running or cycling.
After six weeks, both groups had improved their indicators of exercise capacity (performance time, VO2max, perceived exertion, exercising heart rate, and respiratory capacity) by about the same amount. However, those who trained in the afternoon had a much higher 'ventilatory threshold' (VT) in the afternoon than the morning. VT is simply the exercise intensity at which breathing begins to become laboured and is a fairly good indicator of performance potential: the higher the VT, the better the performance. In fact, the afternoon-trained athletes were able to exercise about 2.5% longer when a rugged endurance test was carried out in the afternoon, compared with the morning.
This does not necessarily mean that the afternoon is the best time to train: the fact that afternoon-trained athletes performed better in the afternoon than in the morning simply suggests that perhaps athletes should try to train at the same time of day they intend to compete. If, for example, you train in the afternoon but race in the morning, perhaps you're not really teaching your body how to perform well in its morning 'state', when the rhythms of muscular efficiency, body temperature, and muscle strength are different. Indeed, studies with weight trainers show that they are most powerful at the time of day when they usually train and less strong at other times. Research with animals also indicates that they have greatest endurance at their usual time of training and less stamina at other times of the day.
Strangely enough, though, in the Georgia study athletes who trained only in the morning performed equally well in the morning and afternoon. So where does that leave us? To gain a better understanding of how athletes respond to training at different times of the day, scientists at the University of North Texas recently asked six athletes to train only in the morning and six others to work out just in the afternoon. After five weeks, morning-trained athletes had about 13% greater endurance in the morning than the afternoon, while afternoon-trained athletes had 13% more endurance in the afternoon. The bottom line? You'll probably compete better in the morning if you train in the morning and vice versa. If your key event is scheduled for 2pm, that's when most of your high-quality training sessions should take place.
The impact of time zone changes
Your circadian rhythms become especially important when you travel across time zones. For example, let's say you live in London and fly three hours east to Moscow on Friday for a race scheduled for 8am on Saturday. When you show up at the starting line, you feel surprisingly lethargic and stiff, even though your training has been going well and you've had a fairly restful week. Of course, the problem is that your rhythms haven't adjusted to the new time zone. When the starting gun sounds at 8am, your body is still on London time and thinks that it's really 5am, when muscle power, body temperature, and flexibility reach low ebb. In such a situation, your performance is very likely to be below par. And in fact studies carried out with fencers at the 1984 Olympics showed clearly that those who didn't have enough time to adjust their body rhythms to local time fared worse in competition than those who gave themselves adequate time to get their bodies 'in synch' with the local time zone.
Fortunately, there are three sound solutions to the time zone problems faced by athletes:
1. Travel to your race site well in advance, giving yourself one day for each hour of time change to adapt to the new time zone. From Los Angeles to New York (or vice versa), for example, that would mean at least three days at the race site before the actual race date, since there is a three-hour time difference between the cities;
2. When you arrive at the race location, immediately adjust your bedtime, waking, and eating patterns to the new time zone. This will help your body adjust more readily;
3. If your schedule doesn't permit lengthy stays at the race site before the event, you should try to train at the exact time of the race. Using the example above, if you live in Los Angeles and want to run an 8am New York race but can't get to New York until the day before the competition, train every morning at 5am in Los Angeles for several weeks before the race. Since New York is three hours ahead of LA, your ungodly 5am sessions will teach your body to function well at 8am in the Big Apple.
Sometimes this kind of solution isn't really feasible, though. For example, let's say you live in London and are planning to run a marathon in Beijing. The time difference is hefty - eight hours ahead - and the race starts in Beijing at 9.30am, ie 1.30am in London. You can hardly get in your 20-mile training sessions in the middle of the night, so the only possible solution is to get to Beijing as far in advance of the race as you possibly can. To ease the time zone adjustment, you should also try to 'get in the swing' of Beijing life as soon as you arrive, rising at 7-8am local time, shunning long afternoon naps and retiring for the night at 9-10pm. To promote adjustment, you could take supplements of melatonin (a natural hormone produced in the brain which seems to induce sleep) on your flight and before bedtime for the first few nights at your destination. It is also a good idea to try to get into bright light as soon as you wake up each morning, and take your meals at regular times of the day at your race destination. Finally, remember to alter your bedtime for the few days before your departure. If you're travelling across time zones in an easterly direction, go to bed earlier; if you're heading west, try to stay up later, but don't lose so much sleep that you end up excessively tired.
The trials of shift working
Getting out of synch with your own body rhythms also happens with night shift working. Your first night on the job is a bit like, say, flying from London to Los Angeles: since Los Angeles time is eight hours behind British time and a typical work night prolongs your waking 'day' by eight hours, in each case you have extended the day by about a third. Internally, your body doesn't quite know how to react. On your first day on the west coast of the USA, the bright, rising California sun tells your brain that it's 7am, time to get revved up for the day, but your body tells you that it's 3pm, time for a brief nap. Similarly, after a night shift, the morning sun, the din of morning traffic and human activity tell you it's 7am, but your mind and body are more than ready to slumber.
Although there are similarities, working at night is actually much tougher on you than crossing time zones: that's because your body never really gets a chance to get in synch with the surrounding 'zeitgebers' (time givers) - environmental factors which adjust your internal body rhythms so that they correspond with the 24-hour period of day and night. Light itself is a powerful zeitgeber; studies have shown that animals exposed to brief blasts of light in the evening are fooled into thinking that days are getting longer; chickens similarly exposed increase their egg production as though it were springtime. Humans respond strongly to light, too, as well as to other zeitgebers, such as social routines, mealtimes, bedtimes and, of course, the familiar ring of the alarm clock.
When you travel across time zones, all your key zeitgebers change together. Even though your body may be crying out that it's three in the afternoon, the rising sun, your breakfast, the noise of traffic, the commotion created by people on their way to work, and your morning meeting with business associates all tell you that it's morning, damn it, regardless of what's happening inside your body. Changing time zones is tough, but gradually you adopt the habits of the people around you.
Shift work is considerably harder because the zeitgebers are always out of synch with your body. Your spouse's conversation, the cereal in your bowl, and the morning light all tell you that it is the beginning of your day, but your all-night work session has left you feeling that it's time to collapse into bed. You are in conflict with all of the zeitgebers around you, and your need to work preserves this conflict. Unfortunately, the resulting disruption of rhythms often makes it difficult to sleep during the day, so you may gradually sink into a deeper and deeper state of fatigue.
How can you minimise the impact of shift work on your workouts? There are several simple things you can do to make your body rhythms correspond with your new work schedule:
1. When you leave work in the morning, avoid bright light as much as possible (because intense light will make your body think that you're at the beginning - not end - of your day). By contrast, when you wake at, say, 4 or 5pm, try to get into light immediately, preferably by going outside;
2. Use an alarm clock to rouse you in the afternoon and try to internalise the idea that this marks the actual beginning of your day;
3. Eat your normal breakfast when you get up in the afternoon, not when you get home from work in the morning;
4. Maintain social contacts: if you were busily involved with other people in the mornings when you worked during the day, aim for frequent social interactions in the afternoon.
Larks versus owls
Bear in mind that if you're a 'lark', the kind of person who leaps out of bed full of energy at 5-6am, you'll find it harder to adjust to shift work than an 'owl', someone who loves staying up late and crawls out of bed each morning in a semi-comatose state. Since owls are natural burners of midnight oil, working through the night feels fairly natural to them, while larks, who are often in bed before 10pm, rebel at the whole idea of nocturnal work. In truth, the circadian rhythms of owls probably extend over 25 or even 26 hours rather than 24, which is why they have less problem extending their days; larks, on the other hand, tend to have shorter circadian rhythms of around 23 hours.
Athletic larks and owls respond quite differently when they change time zones for competition. Owls always fare best when they travel in a westerly direction, since such travel naturally elongates the day, while larks are better with easterly travel, since this brings up the morning sun much more quickly. If you know you're a lark or owl and you have a choice of events to compete in and a limited amount of time to adapt to different time zones, bear these considerations in mind.
Getting back to shift work, let's say you have a race scheduled for 8-9am on a Saturday and you have to work through Friday night. You'll just have to try to tough the race out, even though your body would rather be in bed. Bear in mind, though, that your performance will be about 2-3% slower than usual, and take this into account when reviewing your performance.
If you're forced to work shifts, it's best to reserve your really tough training (including intervals, hill reps, and long runs) for days off - ie those, which are not followed by a night of work. In addition, if you work from 11pm to 7am and sleep from around 8am to 3pm, it's best to train at about 9-10pm, if possible, because that time period will be, in effect, your 'afternoon' - the time when it's possible to train at your most efficient level. If you don't overdo your workouts when you start to work shifts, your body will gradually adjust to your new lifestyle, and your exercise programme will be OK.
Just one final point about body rhythms and athletic performance: body temperature follows a natural circadian rhythm, reaching a high point in the late afternoon and dropping to its lowest level just before dawn. The afternoon peak in temperature is one reason why competitive performances tend to improve then; muscles simply function better when their temperature is raised a bit. Athletes often start an afternoon competition with their body temperatures about 0.5¡C higher than they would be in the morning. So, afternoon efforts can be 'hot' in more ways than one!
Owen Anderson