When the results were tallied up, the researchers saw a range of responses. “Nobody has the same nose,” Dr. Herz said. Some adolescents had only very mild changes in sensitivity, while sensitivity altered dramatically in others.
Averaged together, however, the results showed that overall the circadian clock does affect smell, and that the times when the children’s noses were most sensitive tended to correspond to the evening, with an average peak of 9 p.m.
“The results make sense — the circadian clock affects virtually every organ system in the body,” writes Dr. Leslie Vosshall, a researcher at Rockefeller University who studies smell and was not involved in the study, in an email.
Smell was at its lowest ebb, intriguingly, from about 2 a.m. to 10 a.m.
It is already known that when we are asleep, a strong smell won’t disturb us the way a loud noise or a bright light will. Perhaps the biological machinery behind smell shuts itself down for the night, at least in some people. But Dr. Herz speculates that having stronger olfactory abilities as dusk fell might have helped our ancestors survive.
“It really underscores the importance of auditory fire alarms,” she said.
Still, the experiment was designed to test the effect of the circadian clock, and that is not the only factor involved in smell sensitivity. Researchers have already found that another big player is how long someone has been awake and what variety of smells they have been exposed to. It’s likely that all of these have a role in determining when, in real life outside the lab, our sense of smell works best.