NEW YORK, Aug. 25 -- An investor who sets an alarm for the opening bell in New York has, on a typical day, already missed about half of what the Nasdaq is going to do. The other half happened while they were asleep, in a futures market that most people never look at and that no evening bulletin reports.

Monday was the pattern in unusually clean form. The Nasdaq 100 finished Friday at 29,308.86. It opened Monday at 29,094.72, a fall of 0.73 per cent that occurred entirely overnight, before a single share changed hands in New York. Across the whole six-and-a-half-hour cash session that followed, it lost a further 0.14 per cent. Roughly five sixths of Monday's decline was already done at the opening bell.

That is the case for watching futu...