No, not THAT clock! This one affects both men and women.
The biological clock is an internal timing mechanism that exists in some living systems as a tiny clump of cells in the brain that regulates our daily cycle of sleep and wakefulness. These cells are strongly influenced by the daily change between sunlight and darkness, as well as the seasonal flux of light and the planet's rotation.

Our biological clocks control our circadian rhythms. Wait, whoa, what's that complicated word? Bear with me, this is important to understand why we get sleepy and wake up in a regular pattern. The word circadian comes from a Latin phrase meaning "about a day." Circadian rhythms are physiological and behavioral characteristics that follow a daily, or circadian, pattern. Our bodies display hundreds of these circadian rhythms, including sleep and wakefulness, body temperature, blood pressure, and the production of hormones and digestive secretions.

So what does this mean? Basically, what our circadian rhythms do is "program" us to sleep at night and to be awake in the daytime. For most people, it is difficult to concentrate and maintain alertness between midnight and 7 a.m. and to sleep during the daylight hours. And it's hard to eat in the overnight hours when our digestive systems essentially shuts down. This explains why we don't get hungry and wake up while sleeping, even though we tend to get hungry every few hours while we are awake. The important reason why our biological clock and ciradian rhythms program us this way is so we can have sustained periods of sleep and wakefulness. If we could be "deprogrammed," we would behave more like cats, napping for briefer periods of time and waking up more frequently throughout the day or night.

"But I'm a night person," you protest. People vary in their individual circadian profiles, with some being "larks" and others being "owls." If you're interested in finding out more about circadian rhythms and taking a "Lark and Owl Test,"
check this site out.