The highest downvote to upvote ratios and highest downvote to comment tree depth increase (i.e. discussion happening) occur in the time periods of 12-1 and 3-4 US East coast time by a factor of about 2. Those are actually the only hours of the day when my account personally (the only one I was tracking) has an upvote/downvote ratio less than 1 (ratio was 1.5-2 for the rest of the day). Based on the fact that I would say that there certainly exists a group of users who "reflexively" downvote. An alternate explanation is that people who hate what I have to say are most likely to use HN during those hours. Both those options seems highly plausible to me.
This analysis is about a year old and based on about 6mo of data. I have since lost the script and the records so don't expect any further analysis.