Differences between males and females abound, of course — but some are found in the oddest places. New research has found that women tend to have shorter, earlier sleep cycles then men. This makes women typically go to bed earlier and get up earlier in the morning. It could also cause women's higher rates of insomnia and seasonal depression.

"This has implications for how easily they can fall asleep and how well they can stay asleep," said study researcher Jeanne Duffy of Harvard Medical School. "It could alter and contribute to differences between individuals as to when it's easy to go to bed or wake up."

The researchers found that, on average, women's 24-hour sleep-wake cycle (called the circadian rhythm) is about six minutes shorter than men, but in the reality of sleeping and waking, this equates to waking up about 30 minutes earlier.

Extreme sleeping

The research team, led by Duffy and advisor Charles Czeisler, studied the sleep cycles of 52 women and 105 men for two to six weeks in the lab. They studied two indicators of circadian rhythm, the patient's core body temperature and levels of the hormone melatonin — thought to play a role in setting sleep-wake cycles — while the patients followed extreme schedules (following a sleep-activity cycle spread across a 20 or 28 hour day, instead of the normal 24) in a dimly lit room.

This environment allows the researchers to measure the natural circadian rhythms of the individuals, which are normally reset daily by exposure to natural light. Without outside cues, the body reverts to its natural cycle, which is sometimes longer or shorter than 24 hours. In this study, about 35 percent of women had circadian rhythms shorter than 24 hours, compared to 14 percent of men.

This difference is important for people with seasonal depression, who are treated with light therapy to reset their circadian rhythm. If they have a cycle shorter than 24 hours, they need evening light to sync up, and if it's longer than 24 hours, they need light in the mornings.

Why women?

The finding could have to do with differences in estrogen levels, the researchers say. This could mean that hormone levels could change circadian rhythm, though this evidence on pre- and post-menopausal women suggests that sleep cycles are related to hormone exposure during development, not adult levels.

Figuring out what controls our biological clocks "is one of the most important questions in human chronology research right now," Alfred Lewy, of the Oregon Health and Science University in Portland, told LiveScience. "It's brilliantly done and has important clinical implications," said Lewy, who wasn’t involved in the current study.

The research was published online May 2 in the journal Proceedings of the National Academy of Sciences.

This article was reprinted with permission from LiveScience.

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