Plants gad individual timing to collect "potencies" month time of the day wetter condition etc.look into older botanical books ( befor Internet ) all Wes collect plants in warm afternoon condition .
It is generally recommended to avoid collection between sunrise and sunset (due to the possible degradation of photosensitive components). So you have the choice between the earlier morning and evening. Personally I prefer the morning time.
I think it really depends on the type of your target compound to be extracted. Generally we need to minimize the degradation routes, such as hydrolysis, oxidation, photolysis etc, of our target compounds during collection, prior to extraction, upon extraction and after extraction during storage.