Not drinking enough water is the most common reason that urine turns dark yellow. To find out if this is causing the darker output, try drinking a couple of glasses of water and see if your urine changes to a lighter yellow.
Urine color is an excellent way to gauge if you are drinking enough fluids. If your output is pale yellow or clear, you are probably getting enough water. If your urine is dark yellow that means it is concentrated, and you need to drink more. Be sure to increase your intake if you sweat.
It may take some trial and error to find the best amount of water that is right for you. If your urine is still dark after a day of drinking 8 to 10 full glasses of water, then see your health care provider.