There are four rhythms which correspond to and synchronize with geophysical cycles in the natural environment. Collectively, they are referred to as the circarhythms. Sleep researchers have provided people with an understanding of sleep as an active process. They have variously divided human sleep into stages. For example, in one scheme, individuals are considered to be awake, in rapid eye movement (REM) sleep, or non-REM sleep. The time domain of rhythms with longer than 24-hour periods, or lower frequencies than circadian rhythms, is occupied by infradian rhythms. Probably the most studied infradian rhythms are the reproductive cycles that are found in female laboratory rodents—mice, rats, and hamsters. These animals enter and exit a period of sexual receptiveness called heat, or estrous, during which they are willing to mate and able to conceive. The human menstrual cycle averages 29.5 days between the ages of 15 and 40 years. Cycles tend to be longer than 28 days at puberty and menopause.