A cup of coffee isn’t the only thing that can cause your energy levels to jump around. The other reason why you feel wide-awake at some points of the day and drowsy at others? Your circadian rhythm, an internal clock that helps regulate the cycle of when you feel sleepy and when you feel alert.

In a broad sense, circadian rhythms are similar from person to person, operating on roughly 24-hour cycles. But it turns out there are some notable differences in the sleep/wake patterns of women and men, which could explain why men tend to be night owls while women are more apt to be early risers.

The Rhythm Method

Circadian rhythms are controlled by an area of the brain called the hypothalamus. Other influences include light (which sends a message to your brain that it’s time to wake up) and darkness (an indicator to your body that it’s time to release melatonin, a hormone that helps you fall asleep). Regular sleep patterns—waking up and going to bed at the same time daily—also keep your circadian rhythm functioning normally, helping to reduce the chance of sleep trouble such as insomnia.

What’s Sex Got to Do with It?

Beyond these factors, there’s another important variable that influences your internal clock: sex.

It turns out, male and female circadian rhythms don’t exactly match up. Men’s clocks tend to run truer to a full 24-hour cycle or longer (on average, men have a circadian cycle that’s six minutes longer than for women ) meaning they may feel less tired in the evening.

In women, the internal clock is more likely to be shorter than a full 24-hour cycle, making it more likely that they will awaken earlier, which may also increase their susceptibility to early-waking sleep disturbances like insomnia.

Handling Sleep Cycle Interruptions

While eight hours per night on average is ideal for both genders, it turns out that men are harder hit by periods of deprivation. Lack of sleep causes work performance to suffer more for men than for women, and men recover less quickly from lack of sleep than women do.

On the other hand, women’s shorter cycles mean they are more likely to have a dip in energy at night, which could help explain why there’s an increased risk of work-related injuries in female shift workers.

Of course, it is possible to learn how to re-train your inner clock to help you feel more awake or sleepy at different parts of the day depending on your lifestyle needs. But left to its own devices, the body’s natural rhythms make it more likely that if you are a man, you will be a night owl, and for women, an early bird.