Dry eyes with soft contact lenses and the feeling of dryness on the front of our eye with a piece of plastic on it makes us want to close our eyes or squint. This can lead to a feeling of sleepiness, especially in the afternoon/evening after a big carb lunch or dinner (so food coma can be a factor).
Dry eyes usually occurs more often in dry environments like some offices and cars where the A/C is blasted. Sometimes those environments are also more conducive to sleepiness. Also dry eyes occurs where the moisture content is very low. So, before using contact lenses in such environment and in such conditions, it is better to consult the concerned eye specialist.