Another trading day, another collapse in the final hour. 

The Dow's cumulative loss of 550 points over the past two days has occurred during the last hour of both days.

This is insane. 

These last hour swoons (and gains) have been quite common in this vicious two-month sell-off

So that got me thinking - what have been the intra-day returns?

I broke the day into four time segments - the overnight return (the difference between the market close and the market open), the first hour of trading, the day between the first and last hours of trading, and the last hour of trading.

The worst losses have not been in the last hour of the day.  In fact, the first hour and overnight have both sustained heavier losses. 

Since September 1, the cumulative loss for the S&P 500 during the first hour of trading has been 17.4% and the overnight return has been -12.7% while the last hour has sustained losses of 11.0%. 

Most of the damage has occurred since October 1.  Since the beginning of the last month, the markets have declined cumulatively 15.3% in the first hour, 11.0% overnight and 8.8% in the final hour.

During the middle of the day, markets have actually risen.  Since September 1, the cumulative gain in the S&P 500 has been 3.3% from 10:30am to 3:00pm.  Oddly, during the teeth of the decline since October 1, stocks are up 6.6% mid-day.