Changes in women's bodies occur at different times in the menstrual cycle and may affect sleep. For example, 50% of menstruating women report bloating that disturbs their sleep according to NSF's Women & Sleep poll. On average, these women report disrupted sleep for two to three days each menstrual cycle. These changes can be linked to the rise and fall of hormone levels in the body. In general, estrogen (estradiol) increases Rapid Eye Movement (REM) sleep, a deep stage of sleep when we typically dream. The hormone progesterone, which rises after ovulation (when an ovary releases an egg), may cause some women to feel more sleepy or fatigued. However, poor quality sleep is more likely at the beginning of the menstrual cycle when bleeding starts. Other factors that influence sleep are stress, mood, illness, medications, diet, lifestyle and the sleep environment. Women's sleep experiences vary greatly during the menstrual cycle and certain types of sleep problems are associated with each phase of the cycle:

Before Ovulation (days 1-12): Typically, the period (bleeding or menstruation) occurs for about five days. After the period ends, an egg ripens in the ovary. During menstrual bleeding, women tend to get less restful sleep than at other times. 36% of women polled by NSF said their sleep was most disturbed during the first few days of their menstrual periods.

Ovulation (days 13-14): An egg is released. If the egg is fertilized, pregnancy occurs. Otherwise the cycle continues and a menstrual period results in about 14 days.

After Ovulation (days 15-28): If pregnancy doesn't occur, the lining of the uterine wall begins to break down; it is shed during the menstrual period.

• Progesterone levels start off high at the beginning of this phase, reaching their peak level around days 19-21. Then levels begin to decrease toward the end of the phase. Women may find it more difficult to fall asleep. This may be related to the rapidly falling levels of progesterone.
• Premenstrual Syndrome (PMS) symptoms may occur during the later portion of this phase (starting about days 22-28). These include bloating, headaches, moodiness, irritability and abdominal cramps: any or all may contribute to difficulty sleeping well. The most common sleep-related problems reported by women with PMS are: insomnia (difficulty falling asleep, staying asleep, waking up too early or unrefreshed sleep), hypersomnia (sleeping too much), and daytime sleepiness.