In human beings, a variety of physiological functions such as body temperature and heart-rate undergo distinct rhythmic changes in the 24 hour period. Generally the values are at their lowest during the night and reach their peak in the afternoon. This phenomenon is known as Circadian Rhythms.

These rhythmic variations in the physiological mechanisms of the body are linked to variations in performance. According to Professor T. Reilly (Sports Science advisor for the English FA), there are two major rhythms that have relevance for exercise and sports performance - body temperature and the sleep-wake cycle.

For example, values for leg and back strength, jumping performance, flexibility and anaerobic power are at their highest in the late afternoon (following the body temperature curve). It seems that physical conditioning work best takes place in the afternoon leaving the morning session for skill work especially as arousal levels and activities depending on the central nervous system tend to peak around midday. A graded warm-up is useful in the morning to avoid stiff joints and muscles (not yet at their optimum temperature) being injured.

Generally, Circadian Rhythms do affect performance and the coach must take this into account when measuring physical performance and coordination skills as well as in daily training and competition.

Sleep plays a prominent role in preparation for training and competition. Individuals vary in the amount of sleep they need and research by Reilly showed that professional players do spend more time in bed and resting than the average adult. The effects of sleep loss depends on the type of task. Challenging or difficult activities are less likely to be influenced by reduced sleep. However, consecutive nights where the player has slept badly (partial sleep loss for example) will affect the players ability to perform. Other factors such as alcohol and heat can interact with sleep loss to further deteriorate performance.


Humans often experience travel fatigue during long journeys. Even without crossing different time zones which brings about jet-lag, fatigue can occur due to boredom or stiffness from unfamiliar seating/poor posture. Teams may want to travel the night before to avoid fatigue although sleeping in an unfamiliar environment can be detrimental to players who do not feel comfortable away from home. A refreshing shower along with light exercise and stretching can relieve any lethargy.

Jet-lag arises mainly from crossing time zones due to biological rhythms being disrupted. The body's rhythms on arrival try to retain the characteristics of their point of departure. However, the new environment forces new influences on these cycles, the main factors being the time of sunrise and onset of darkness. The body attempts to adjust to this new context but core temperature does not adapt very quickly. The causes can as well be linked to:

Cabin dry atmosphere/stale air/air pressure: A dry cabin helps increase the chances of headaches and dehydration, dries the skin and dry nasal and throat membranes thus creating good conditions for catching colds, sore throats etc. Stale air as well as cabin pressure can lead to tiredness, make you irritable and give headaches.

Food & Drink: Drinking a lot of tea, coffee and alcohol can accentuate the effects of dehydration. Also, according to the World Health Organisation, 50 % of travellers get stomach problems so good dietary care is vital.

Lack of exercise: People often get stiff and uncomfortable when travelling due to being sat for long periods, this increasing their general discomfort.

Pre-flight condition: If you are already tired, nervous, over-excited or hungover then this will increase the effects of jet-lag.

Direction of travel: This affects the severity of jet lag. It is easier to cope with flying in a westward direction as the body's rhythms can catch-up quicker when the day is artificially lengthened due to the natural circadian rhythm being longer than 24 hours. American football teams can be at a disadvantage when travelling coast to coast. Even a three hour time difference has been shown to adversely affect performance.

Lack of sleep: A non-familiar environment may result in players not being able to sleep on a plane which will obviously leave them tired.

The symptoms of jet-lag on arrival include being tired for days, sleeping problems as well as a loss of drive/motivation/concentration and appetite. Even simple tasks can prove difficult. Evidence shows you need one day for every time zone crossed to regain normal rhythm and energy levels. However, some individuals show little or no effect of jet-lag and fitter or younger people seem to be less affected.

There are many possible ways to reduce the effects of jet-lag:

Before & During Travel: Planning your trip such as the departure and arrival time and preparing everything so you are not rushed may help. Some travellers believe jet-lag is reduced by travelling in the day. Athletes should try to arrive well in advance for competition. Try to arrive in top form meaning after a good nights sleep. Drink a lot of water to reduce the effects of dehydration and avoid beverages such as coffee and alcohol. If you can, take a shower as this will freshen you up, tones the muscles and gets the blood circulating again. Also, try a few light stretching exercises as well as walking around the plane to prevent stiffness.

Sleeping pills may help you to sleep but can have secondary effects such as drowsiness and dehydration. Plus recent studies show that people who stay static and sleep in their chair are more at risk of developing blood clots. Diet, drugs and light exposure are other factors used to reduce jet-lag.

On arrival: A key factor for the athlete is to fit in immediately with the phase characteristics of the new environment. Light exercise even on the day of arrival can be beneficial as travelling eastward will delay the onset of sleep although going to bed early can also help alleviate fatigue after travelling westward. The severity of symptoms may be worse 2-3 days after arrival so the athlete and coach should be prepared for any further deterioration in performance.

Exercise is more useful to help re-tune rhythms than taking naps which can anchor rhythms at the zone of departure. Training in the morning may be beneficial. Again various drugs are available on the market to help adaptation. The individual may have difficulty in sleeping for a few days, but activity and social contact during the day will help in accelerating adaptation.

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