In the spring a young man's fancy lightly turns to thoughts of . . . bugs.

By pinpointing DEET's molecular target in insects, researchers at Rockefeller University have shown that DEET acts like a 'chemical cloak', masking human odors that blood-feeding insects find attractive. This research makes it possible to improve the repellent properties of DEET and also make it a safer chemical.

"For all these years, there were a lot of theories but no consensus on how DEET worked," says Leslie Vosshall, head of the Laboratory of Neurogenetics and Behavior. "Does it smell bad to mosquitoes or does it blind them to odors" It was a great unsolved problem."

Mosquitoes are strongly attracted to odors in human breath and sweat, including carbon dioxide, lactic acid and an alcohol-based compound called 1-octen-3-ol. Different receptors within their olfactory system detect these odors, among others, and lead them to their prey. . .

Although DEET is widely used, concerns about its potential health risks have prompted scientists to pursue alternatives, though so far none have proven to be significantly more effective than DEET. "We now know how DEET works, and this is the first step in making significantly better insect repellents," says Vosshall.

OK, not just bugs.

The biological clock, called the suprachiasmatic nucleus (SCN), sits in mammals' hypothalamus. It monitors light through a pathway to the retina and conveys information about day length to the pineal gland. This pea-size gland, tucked at the base of the cerebrum, controls the secretion of melatonin, dubbed the sleep hormone because it is only released in the dark or in dim light. The duration of melatonin release changes with nocturnal length, which is longest during winter. And it has been thought that our increased energy in the spring months is somehow linked to the decreased duration of melatonin production, due to shorter nights.

"From a biological perspective, most types of animals, and maybe even plants, have seasonal variation in behavior and physiology; there are seasonal cycles in human rates of conception," says Thomas Wehr of the National Institute of Mental Health, who reviewed the effect of biological rhythms on reproduction in 2001 for the Journal of Biological Rhythms. Historically there have been more births in the spring. In the late 16th century birth rates typically spiked to 20 percent above the average in March—meaning the babies were conceived in June—but over the past 400 years rates have flattened to about 10 percent above the average, according to research done by David Lam at the University of Michigan's Population Studies Center in Ann Arbor.

Cultural and social factors influence conception patterns but biology plays a strong role, as shown by peaks that are 20 percent above average during June—technically the tail end of spring—in the production of reproductive fuel: luteinizing hormone, which produces testosterone in men and triggers ovulation in women. Research also shows that successful in vitro fertilization follows the same seasonal peaks as natural birth. "In humans we don't know for sure what the causal connection is," Wehr says, "but if most other mammals are using changes in day length, then the melatonin signal and conception rates is a pretty plausible relationship, but more research is needed."

Latitude matters. The further from the equator, the greater the effects.

P.S. It's Pi day Friday. That only happens once in a blue moon. So to speak.