The underlined part "jet lag" in Paragraph 6 most

单项选择题

Are you an early riser or a night owl
Researchers from Aachen University in Germany believe that about 10 percent of people can be classified as "morning people", who feel more active and function best in the morning. Around 20 percent are night owls—people who naturally tend to stay up late and are more tired during the day. And the rest of us fall somewhere in between, according to New Scientist.
Previous studies have suggested that early risers are more likely to be happy and healthy while night owls experience worse sleep as well as more depression and anxiety during the day.
A new study suggests that it isn"t just people"s habits—early risers and night owls actually have different brain structures. For a long time, scientists have been trying to find out what causes the differences between the two.
Led by Jessica Rosenberg at Aachen University, researchers scanned the brains of 16 early risers, 23 night owls and 20 people with intermediate sleeping hours. They found that the brains of night owls had less "white matter"—which speeds up the transmission of nerve signals—in brain areas associated with depression.
As you know, after people fly in an airplane from one time zone to another, they often suffer from a confused and tired feeling called "jet lag" because their body clocks are out of sync with the new time zone. It usually takes about a week for their bodies to adjust to the new time. But night owls always have difficulty syncing their bodies to the right time due to their brain structure. "It"s like they suffer from permanent jet lag," said Rosenberg.
The good news is that it is possible for night owls to turn themselves into morning people. According to the researchers, night owls should try to spend as much time in the sunlight as possible and reduce their exposure to artificial light at night to force their body clocks to shift to a more normal rhythm.

The underlined part "jet lag" in Paragraph 6 most probably means "______".

A．the delayed flight of jet
B．the suffering from slow flight of jet
C．the physical tiredness through lake of sleep after a long flight
D．the tiredness and confusion caused by flying across several time zones