The researchers also found that a high carbohydrate regime reduces some, but not all, of the effects of hard training.
For the study, the scientists from Loughborough University in England studied the effects of two nine-day periods of heavy training on 13 highly trained cyclists.
The researchers monitored the athletes' moods, sleep patterns and performance before, during and after exercise.
To determine whether diets could counter the effects of any sleep deprivation, the athletes were also given high or moderate amounts of carbohydrate throughout the study, though none of them knew which.
The researchers discovered that even as little as nine days of intense training can cause 'significant and progressive decline in sleep quality'.
"Sleep efficiency was significantly reduced during the intensified training period," the researchers observed, with the number of times the athletes woke throughout the night significantly increased.
They also noticed that the athletes' moods and capacity for exercise both worsened over the period of observation.
As for the additional carbohydrates, the team concluded that a high carbohydrate regime reduced some, but not all, of the effects of hard training.
The study appeared in the Journal of Sports Sciences.
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