A research study was done on 21 boys and 16 girls, all between ages 12 and 15. Researchers measured their 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 then conducted a sniff test to determine their threshold of smell. They found 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. The study suggests that sensitivity might be sufficiently higher in the afternoon rather than night. On average, the peak of smell sensitivity was at the beginning of biological night, or about 9 p.m. for the teens. From an evolutionary standpoint, this might be to ensure the greatest sense of satiety during the important end of day meal, it might be a way of increasing mating desire, or perhaps a way of scanning for nearby threats before bedding down for the evening.