Personally, I think time of day doesn't matter so much as light levels, be it early morning or from late afternoon...certainly all the decent roach I've caught have come on days when heavy cloud conditions, along with mild weather have been the order of the day. NOT so say big roach cannot be caught in bright sunshine, but IMHO I prefer the latter, if I have a choice. Having said that, I have also sat fishless in what would be generally described as 'perfect' conditions.....
However, as 'robertroach says above, I believe roach fishing with hemp and tares produces decent fish on bright, but not sunny days - at least it's the case on the three fisheries I fish on a regular basis !
I've long given up trying to work around weather patterns as a guide to success as I fish so often that I have to work with what the day brings. I have gone from a reasonable bag of roach last Sunday to a single roach midweek ( bits don't count !) from the same swim - with conditions very similar apart, from a little more wind !!
I would say, (very generally speaking !) that starting to fish from very early morning your chances of big roach (possibly ) recede as the day wears on, whereas fishing from late afternoon into evening the chances (possibly ) increase, but then someone else will think precisely the opposite, and that, as they say, is why we go fishing in the first place !!!
Goodness knows what tomorrows heavy frost will bring......................................