I have found you are best off to fish early in the morning, and late in the evening. Having been doing this for a long time, I can say it is often dramatic what a difference if makes. The difference is sometimes lessened on a cloudy day, but on a sunny day, you are much better off taking a mid day nap and resting up for an evening of fishing. Someone may be able to come up with an exception, but day in and day out, fishing the low light times of day is the way to go.
So wrote Rudyard Kipling - “Only mad dogs and Englishmen go out in the noon day sun”