According to the text, the wrong alternative is

SIESTA TIME
    Finally, vindication for power nappers. Far from being lazy louts, siesta-takers are actually doing their bit for the firm. According to Sara Mednick and her colleagues at Harvard, just 60 minutes of shut-eye in the middle of the day can make you perform like the fresh daisy you were first thing in the morning. But it has to be bona fide sleep; a mere rest, they found, has no effect.
    Dr. Mednick, whose results have just been published in Nature Neuroscience, wanted to know what effect power napping would have on people’s visual perception. She asked 30 student volunteers to come into her laboratory. Four times on the same day, at 9am, noon, 4pm and 7pm, they were required to stare at a computer screen for an hour. Their task was to pick out a vertical or horizontal bar from a striped background - an established test of visual perceptiveness. The more quickly they picked out the bar, the more acute their perception.
    All the volunteers had slept well in the days before the test, and had been warned off alcohol. During the test day, nicotine addicts were allowed to indulge their habits, but everyone had to remain uncaffeinated. Despite this cosseting, the performance of the ten volunteers who went straight through the day without a nap deteriorated rapidly. Their best scores were first thing in the morning, and it was downhill from there on. By the last session, they were taking 52% longer, on average, to identify the orientation of the bar than they had in the first.
The Economist

Volunteers who did not have any sleep during the day didn’t do well on the test.

The last session the volunteers were exposed to was generally longer than the first.

Nature Neuroscience probably publishes scientific work.

First thing in the morning people usually perform well.

The volunteers had been sleeping well before the test.