During April, the S&P 500 Index (SPY) has opened lower 11 times, creating a gap to the downside. On eight of those eleven days, SPY has traded higher from open to close.
Interestingly, during 2009 prior to April, the market traded in the same direction as its opening gap 41 times and in opposite directions 29 times. In April, it seems, there has been a mindset of buying stocks on overnight weakness.
We can think of this as a measure of resilience: how well a market absorbs--and ultimately reverses--overnight weakness. A converse measure of vulnerability would be day timeframe selling following overnight rises.
During 2009 overall, the correlation between the overnight market move and the move during the day session in SPY has only been .16. This suggests that the two time periods trade relatively independent of one another. A big part of what might sustain bull and bear moves is resilience and vulnerability--the tendency of buyers to treat overnight weakness (and sellers to treat overnight strength) as opportunities to get better prices.