Anyone who's claimed "love at first sight" might have her L words mixed up. When it comes to instant attraction, your brain—not your heart—kicks your desire into high gear.

0 to 5 seconds
You spot a hot stranger, and before you can even say hello, your brain has processed his voice, face, and pheromones (the less like you he smells, the more aroused you'll become).

Your brain's ventral tegmental area lights up and begins churning out dopamine, the same chemical responsible for feelings of elation—and the addictive high that comes with certain drugs. (Exercise also increases levels of feel-good dopamine. Experience a runner’s high with our 10-Week Half-Marathon Training Plan.)

At the same time, the areas of your mind that handle negative emotions are suppressed, allowing his irritating habits to go virtually unnoticed.

If you're ovulating, you may be drawn to more masculine traits like a chiseled jawline or deep voice. If you have your period, a guy with softer, more feminine features is more appealing.

The spot deep in your brain that deals with memories is recording his every move and feature, subtly comparing him with past loves. Cue: "He looks just like my first crush..."

After 5 minutes
Your brain has revved up its hormone control center to shoot out chemical signals to your ovaries. The message: "This guy is hot!"

The brain also signals the adrenal glands to pump out adrenaline—thus your pounding heart, sweaty palms, and intense focus. (Do you love an adrenaline rush? Resolve to try one of these 10 Fitness Adventures in 2013.)

Meanwhile, your body is busy producing testosterone, the male hormone often associated with aggressiveness and risk taking. After all, it's hard to flirt when you're feeling shy.

After 10 minutes
While you're swooning from the potent cocktail of adrenaline, dopamine, and testosterone, your brain prompts the pituitary gland to produce oxytocin, the hormone thought to promote bonding—and monogamy. (Powerful chemicals can control how your body responds to certain situations. Learn How Hormones Affect Your Body.)

Within a few months
If you're truly smitten, your brain steps up its production of nerve growth factor, a protein that may increase mental capacity. Sadly, it ebbs back to normal after a year of commitment. (Want to spice up your relationship? Try these 7 Ways to Prolong the Honeymoon Phase.)

Newfound passion might also trigger the adrenal glands to shoot out the stress hormone cortisol. But unlike the cortisol that comes with most chronic stress, this short-term version makes you feel all warm and fuzzy and can also boost your arousal.