According to a new study, Researchers who are looking at how sleep may influence eating patterns in teens. A person's ability to smell may vary throughout the day in accordance with their circadian rhythm. It has always been apparent that some individuals have a better sense of smell than others, but a new study of 37 teens provides the first direct evidence that within each person, smell sensitivity varies over the course of each day. It tracks with the body's internal day-night cycle, or circadian rhythm. This study got published in Chemical Senses.

As one of the five senses, smell is an important ability, Herz noted, not only for experiencing and enjoying the world, but also for receiving information about danger, such as nearby fire or spoiled food, and for basic functions like eating. Changes in the sense during the day can affect all these capabilities. Smell is associated with food consumption, notes Herz, who has authored the upcoming book "Why You Eat What You Eat." So the researchers devised an experiment to determine whether smell varies with circadian rhythm.

28-hour 'days'

To conduct the study, the researchers asked the 21 boys and 16 girls, all between ages 12 and 15, to sleep on a fixed schedule for two weeks before reporting to the Bradley Hospital sleep lab. All along, they lived indoors in dim light, socializing and participating in fun activities with each other and staff members. The goal was to separate them temporarily from typical sleep disruptions and from external cues of circadian timing.

In this way, Carskadon said, their inherent, internal circadian rhythms could be measured, as could the sensitivity of their sense of smell at all times throughout their rhythms. The team measured circadian rhythm by detecting levels of the sleep-cueing hormone melatonin in their saliva. Melatonin secretion begins about an hour before the urge to sleep hits. They assessed smell sensitivity using "Sniffin' Sticks," a common test for measuring odor detection thresholds.

The rhythm of smell

Individuals varied substantially in how much their smell sensitivity varied over a circadian cycle and in when it peaked. But there were clear patterns individually and overall. One was that the variance showed a circadian rhythm, and the other was that smell sensitivity was never strongest well into the "biological night," or the period well after melatonin onset when people are most likely to be asleep and least likely to be eating. In clock terms, it's from about 3 to 9 a.m.

Carskadon said, "The sense of smell changes across the 24 hours of the day.” "We don't know if that difference will affect what or how people eat. There is more to come." He said the circadian fluctuations of smell sensitivity help determine food choices and eating behaviors among teens.