How's your Natural Circadian Rhythm - The Optimal Workout

Everyone knows a "morning person"-someone who is always bright-eyed and chipper first thing in the morning. Everyone also knows an "evening person" -someone who can barely drag out of bed in the morning and is sluggish for the first several hours of the day. For both morning and evening people the tables turn late in the evening: Morning people can barely keep their eyelids open, while evening people are full of energy.

Whether you are a morning person or an evening person is determined by your internal biological clock, called your circadian rhythm (from the Latin circa dies, meaning "about a day"). For some people their circadian rhythms dictate that they are best in the morning; for others, best in the evening. Not surprisingly, circadian rhythms can affect lifting performance. Knowing your circadian rhythm can help you get more out of your workouts.

What's Affected

A number of physiological functions roughly follow a 24-hour cycle and are presumed to be under circadian control. These include sleep, body temperature, strength, arousal, heart rate, blood pressure, urinary excretion and hormonal output. For example, resting heart rate and blood pressure show daily maximums in late morning or early afternoon and daily minimums from about 2 to 4 a.m. This may be related to daily maximums and minimums of certain hormones, such as Cortisol and epinephrine (adrenaline).

The increase seen in resting heart rate in late morning or early afternoon also translates into an increase in exercise-induced heart rate. So your heart rate will get up to higher levels while you're doing submaximal exercise in late morning or early afternoon compared to the same amount of exercise done at other times of the day. Those who gauge their aerobic exercise by following their heart rates need to be aware that circadian changes in heart rates may throw off their exercise measurements.

Body temperature varies by about 1 ° F over the course of 24 hours, with the highest from about 4 to 6 p.m. and the lowest from about 4 to 6 a.m. Two factors relating to getting rid of excess body heat-sweating and blood flow to the surface of the skin-show maximums and minimums at about the same times as body temperature. These circadian variations probably have less influence on performance in the heat than do other factors, such as ambient temperature and clothing.

One circadian-controlled factor that does significantly influence lifting, though, is arousal. Arousal includes feelings of mood, alertness, vigor and overall well-being and probably shows more variation from person to person than most factors under circadian control. Arousal generally shows one peak in the morning, a drop-off after lunch and then a second, greater peak in the afternoon. Arousal may affect lifting in that exercise is generally perceived to be more strenuous and fatiguing when arousal is low and less strenuous and fatiguing when arousal is high.

There is some evidence to suggest that flexibility and strength are greatest in the late afternoon or early evening. If this is true, then, all else being equal, you may get in a better workout by hitting the gym after work than before work. Other factors affecting performance, such as maximum oxygen uptake (a measure of aerobic capacity) and pain threshold, have shown no clear circadian pattern.

Performance

Since such factors as heart rate, body temperature, arousal, flexibility and strength are under circadian influence, it is reasonable to wonder if sports performance as a whole might be influenced by circadian rhythms. Several studies have looked into this.

In one study 16 swimmers, six runners, three shot putters and four rowers were tested for maximum performance in the morning and in the evening. All athletes competed at the intercollegiate or Olympic levels. Nine of the 16 swimmers, all six runners, all three shot putters and all four rowers performed better in the evening (5 to 7 p.m.) than in the morning (7 to 9 a.m.). In another study results of all-out swimming performed at five different times of the day showed steady improvement from 6:30 a.m. to 8:00 p.m., with an overall improvement of 2.5 to 3.5 percent. In a third study subjects tested on stationary bicycles at either 6:30 a.m. or 10:00 p.m. showed significantly greater tolerance for intense exercise, performed more work and produced more lactic acid (a measure of how hard the muscles are working) in the evening than in the morning. Several other studies have reported similar results.

Thus, it appears that sports performance progressively improves throughout the day.

But what of morning people and evening people? How do their circadian rhythms specifically affect their performance?

Experiments that have shown improved sports performance later in the day have generally shown that performance parallels circadian variations in body temperature. On average, body temperature peaks from about 4 to 6 p.m.; however, people who claim to be morning people hit peak body temperature about 70 minutes earlier than do evening people, according to one study. Morning people may, therefore, be at their lifting "prime" somewhat earlier than evening people.

Morning people produce more of the hormone epinephrine in the morning; evening people produce more epinephrine in the evening. Since epinephrine is one of the hormones contributing to arousal and arousal affects performance, having higher circulating levels of epinephrine may allow morning people to lift better earlier in the day.

It has been estimated that the peak time of day for performance by an evening person is two to five hours later than peak time of day for performance by an "intermediate," one who is neither a morning nor evening person. To bring out peak performances, then, athletic competitions are best scheduled in late afternoon or early evening. For evening people they are best scheduled even later than that.

Athletes crossing several time zones to attend a competition should try to minimize jet-lag changes, going to bed earlier or later in preparation.

This same technique can be used by an athlete to prepare for an event scheduled at the biological "wrong time of day" (e.g., a morning posedown for a evening person). Be aware that setting the biological clock back (by getting up earlier) is not nearly as easy as setting it forward (by going to bed later).

If you have a choice, it is probably advantageous to work out and compete during a time that is in concert with your natural circadian rhythm.