Menopausal hot flashes are more likely to fire off during the first half of the night, especially if the room is warm, investigators reported here.

People are more easily roused in the first half of the sleep cycle, whereas rapid eye movement (REM) activity suppresses thermoregulatory signals in the second half of the night, reported researchers at the North American Menopause Society meeting here.

If women experience these symptoms, turning the thermostat down to approximately 64° F for the first half of the night may reduce hot flashes and improve sleep quality, said Robert R. Freedman, Ph.D., of Wayne State in Detroit.

“Hot flash-induced sleep disturbances appear in the first half of the night, when REM sleep is less frequent and is not there to suppress hot flashes,” said Dr. Freedman. “Some reports of poor sleep during Menopause may be due to normal aging, but misattributed to hot flashes,” he said.

Dr. Freedman and co-investigators conducted their study because despite epidemiologic studies and the widely held belief that hot flashes disturb sleep, they had evidence to the contrary from a recent sleep study of their own. In a probability sample of 589 subjects, the team found that postmenopausal women actually have better sleep quality than premenopausal women.

Therefore, they launched a sleep study involving 36 women. Eighteen were postmenopausal and experiencing vasomotor symptoms; six were postmenopausal and asymptomatic, and 12 were still cycling. On four consecutive nights the investigators varied the ambient room temperature. The first night, the adaptation night, the temperature was 23° C (73° F) or neutral. On subsequent nights, the room was randomly 18° C (64° F) or cool, 23° C (73° F) or neutral, or 30° C (87° F) warm.

The investigators particularly wanted to check the relationship between hot flashes and the time of night of awakening. Because most REM sleep occurs in the second half of the night, the investigators analyzed the data by halves of the night.

Among the 36 participants, 13 were eliminated because of sleep disorders, first diagnosed in the sleep study, hot flashes in presumably asymptomatic women, and previously undetected medical conditions such as hypertension. Among the 23 remaining women, the investigators detected “no group differences on any measure when analyzed by the whole night.”

In the first half of the night, however, the women overall had an average of 2.2 hot flashes with neutral or warm ambient temperature. With cool ambient temperature, the flashes also cooled to an average of 1.5 occurrences.

The second half of the night was stable regardless of ambient temperature, with women having an average of 2.1 hot flashes in the second half of the night. Also, all the women had more frequent REM activity during the second half of the night.

Among those with hot flashes, the flashes themselves awakened them in the first half of the night. In the later hours, the women tended to awaken first, and then experience the hot flash.

The team concluded that “some reports of poor sleep during Menopause may be due to normal aging, but misattributed to hot flashes.” Because of the high number of women screened out for previously undiagnosed Sleep Disorders, the findings also suggest that “reports of poor sleep may also be due to other disorders, such as apnea, restless leg syndrome, or fibromyalgia.”