NEW YORK (CNN/Money) - More often than not as we head into August markets get into a muddle, where it can be difficult to figure out what's driving them. Or, in fact, whether anything is driving them at all.

It can be a difficult time for investors -- dangerous, if they're not careful.

It's not hard to figure out why the market tends to head into a funk in the late summer. As second-quarter earnings season winds down, plenty of investors pack up the family in the car, put Barney into the CD player and go. And this is a global phenomenon. In Japan, their heading home for Obon; in France it's Aout Vacance. And suddenly, there is a dearth of new ideas, old ideas exhaust themselves and markets becomes themeless.

Justin Lahart, senior writer at CNN/Money, talks about late summer and the sleepy time in the markets. Play video (Real or Windows Media)

Sort of like what's beginning to happen now. Sure, people are enthusing about how well stocks have been doing, but look at a chart of the S&P 500 and you'll see they haven't made any headway since early June. Yes, second quarter earnings were quite good, but maybe investors are already looking at that as old news, and wondering what third-quarter earnings are going to look like. It's still too early to have a clear picture on that.

Meanwhile, a major linkage between global bond and currency markets has become undone. For much of the year, buying bonds, and betting against the dollar was a great trade. In early June it reversed itself, with the dollar heading higher and bonds falling. But now the two markets are no longer tightly correlated. It appears, said HSBC currency strategist Marc Chandler, that a new theme may be emerging in the market. And what is that theme? No clue.

It's the sort of situation that keeps even the investors who are still coming into the office uninvolved in the market. Why put a position on if you don't know what's going to happen? Which only serves to thin trading volumes -- making market movements even more choppy and trendless.

The danger? August is a sleepy month -- except when it isn't. Often investors get lulled into thinking that nothing is going on and so don't pay attention when things really start happening. When the Russian debt crisis got brewing in the summer of 1998, many market players downplayed its importance -- much to their later chagrin.