The passionate, hot days of summer are over and the cool breeze of fall has set in. We tuck away our bathing suits and summer dresses and replace them with bulky sweaters and slim leggings. Although we're not scantily clad, and spend more time indoors, autumn actually revs up our sex drive.

Unlike many other primates, humans do not have an official mating season. For most female mammals, an estrous cycle — when they're "in heat" — causes behavioral and physiological changes that can make them more sexually active only once a year to several times a year. However, women can be sexually active and fertile all year round — this explains why we don't have the biological urge to settle down in the colder months.

Yet, science has proven our hormones go through seasonal changes that can affect our sex life. During fall and winter, temperatures start to drop, and we have a need to keep warm, often yielding the desire for romance.

The shift from the summer to the autumnal equinox affects our romantic relationships in five distinct ways.

Testosterone Levels Rise

The leaves are falling down, and testosterone levels rise during the seasonal shift from summer to fall. Testosterone levels and sperm counts, which are associated with sex drive, peak in late fall and early winter, initiating sexual desire. In a 2003 study, researchers tracked men's hormone levels throughout the year, and found there's a surge in testosterone. This boost also applies to women and is found to increase libido. Researchers speculate the rise in testosterone levels is either due to decreasing daylight or social activity.

Fertility Rises

The greater the testosterone levels, the greater the sexual desire, the higher probability of high fertility. Typically, sperm concentration and count are lowest from August through October due to the summer heat, and rise when the weather starts to cool, according to a 2010 study. This means it's prime baby-making season.

Seasonal changes in hormones, like testosterone, influence fertility, and therefore, the timing of conception. There's a notable seasonal birth pattern: August and September are the two months with the highest birth rates. This means November and December are the most fertile months of the year.

Boost In Sexual Arousal

High testosterone levels; high sperm concentration and count; and high fertility all leads to increased sexual desire. Surprisingly, the smell of pumpkin combined with other odors can also trigger arousal in men. A 2014 study found the combined odor of lavender and pumpkin pie led to the highest increase in penile blood flow by 40 percent.

Meanwhile, the odor mixture of pumpkin pie and doughnut led to a 20 percent increase in blood flow. Cranberry odors only led to a five percent increase in blood flow. The odors could induce feelings of warmth and nostalgia, or males may have evolved to be sexually primed by food, and the harvest would have been a good time to find a mate and procreate.

Men See Women As More Attractive

Summertime seems like the season men would find women the most attractive, but science suggests otherwise. High testosterone levels in women can lead to thicker waists in relation to hips in the fall. As a result, men perceive women's bodies as more attractive in the colder months. In a 2008 study, 114 heterosexual men were asked to assess the attractiveness of women throughout the seasons. The highest scores for attractiveness were given in the winter and the lowest in the summer. This is brought on by the "contrast effect'; men increase their 'attractiveness criteria for women's body shape and breasts in the warmer months. However, this shifts in the colder months when there's less exposure.

Boost In Watching Rom-Coms

Cold weather makes us want to "hibernate" and stay indoors, increasing the desire for warmth and company. Physical coldness could activate the need for psychological warmth, leading to an increased interest in romance movies and novels, according to a 2012 study. As the temperature drops, we have the overwhelming need to watch Bridget Jones’ Diary, The Notebook, and even When Harry Met Sally.

The colder months may not allow us to play outdoors, but we’re more likely to shack up indoors.