I always figured mainly based on work schedules of requestors, lunch time, other things like that. But I've never put much thought into it aside from being bummed when things come to a halt.
I wonder if that's due to the type of HITs that you seek out.
What types of HITs do you all do primarily? Surveys, Batches, oddball tasks, high paying minimal number of hits, low paying higher number of hits, etc. Masters, non masters?
It usually gets slow for me around 1-2pm EST for a bit, and then finally dies (or I get tired enough) by 7-10pm EST. I typically due a combo of surveys and batches, leaning toward surveys due to lack of quals at the moment.