Q: I feel tired and run down after spending hours and hours on a long-distance flight. What causes this?

It may seem logical to blame jet lag or cabin fever. But for some people the discomfort may arise from cabin pressure.

The altitude for a typical trans-Atlantic flight is 35,000 to 39,000 feet above sea level. Most commercial aircraft cabins are pressurized to the equivalent of about 5,400 feet to 8,000 feet above sea level. Healthy adults can tolerate cabin pressures up to the equivalent of 8,000 feet without serious consequences, but that does not mean there is no effect at all.

In a study in The New England Journal of Medicine, scientists recruited 500 healthy volunteers and simulated the effects of a 20-hour flight at barometric pressures corresponding to 650 feet to 8,000 feet above sea level. The researchers found that blood oxygen levels fell as the altitude climbed, dipping as much as 4.4 percent at pressure corresponding to 8,000 feet. Between 7,000 and 8,000 feet, the subjects were also more likely to experience headaches, fatigue, muscle cramps and back and stomach aches. The symptoms resembled acute mountain sickness in some of the subjects.

Q: My wife easily finds four-leaf clovers, standing up. I never can, even on hands and knees. Why?

The standard strategy involves pattern recognition, “finding a square in a sea of triangles,” said Dr. Marc J. Dinkin, director of neuro-ophthalmology at New York-Presbyterian Hospital/Weill Cornell Medical Center.

A complex neural network is involved, he said. First, the viewer must have the visual acuity and the sensitivity to color and contrast to perceive leaves as separate objects, or at least see the usual white lines perpendicular to each leaf’s axis.

“Assuming the retina and optic nerve are healthy, these functions are likely intact, so why does he have trouble spotting those lucky leaves?” Dinkin asked. “The answer may lie in his visual cortex.”

Once information from retinal cells reaches the visual cortex in the brain’s occipital lobe, it is parsed and sent to centers with specific cells for various orientations of lines. In combination, they detect shapes. From there, data about objects and patterns is forwarded to the temporal lobe for comparison with known images.

Dinkin said the primary issues are probably attention and technique. Attention is crucial, focusing only on key shapes and ignoring superfluous details. As for technique, he said pawing on hands and knees may be doomed to fail; standing back and scanning is more effective, and speed probably improves with practice.