wayne, from what i have read, the bees never 'sleep', they work 24~7 whether it's night or day. as far as the feed is concerned, my guess is the reason they are taking less at night is the bees are busy removing the moisture from the syrup stored during daylight hours during night time hours....?
as far as what foragers are doing, great question, read this somewhere, will look it up!
a good post, and perhaps apis can add his thoughts.