are the regular changes in biological processes that follow rhythmical patterns over a 24‐hour period. These rhythms involve, in addition to sleep‐wake cycles, changes in other body functions, such as the rise and fall of body temperature and blood pressure. Although connected to alternation of night and day, the rhythms are retained (although in 25‐ rather than 24‐hour periods) even when people are forced to adapt to unusual cycles of light and dark, such as those in Antarctica. Animal species other than humans also maintain circadian rhythms, but there are species differences. Horses, cattle, and cats, for example, spend much more time sleeping than do humans. But of particular interest to psychologists are the effects of circadian rhythms on human response to environmental stimulation (that is, on levels of consciousness).