Thank you very much for the useful information David.
Well this change in color of urine is absolutely normal.
The color of urine is due to presence of bile pigments, and varies greatly depending upon the state of hydration of the body.
In winter, every one passes a diluted white urine (since there is no sweating or reduced loss of water from breathing in winter).
In summer, excessive sweating coupled with increased insensitive loss of water from body (during breathing) leads to reduced water content of urine (thus making it concentrated) and hence a dark color.
Similarly the urine which we pass early in morning after a long nap is always a little dark compared to the one which we pass in afternoon (when hydration status is better).
So in the absence of any symptoms, this is completely normal.
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