The regular cycle of sleep and wakefulness in our lives is an example
a biological rhythm, one of many periodic behaviors exhibited by hu-
mans and other organisms. Sleep and wakefulness follow a circadian
pattern (Latin circa = about, dies = day), having a periodicity of ap-
proximately twenty-four hours. Other examples of circadian rhythms
are body temperature variations and synthesis of cortisol and other
glucocorticoids by the adrenal gland. Melatonin is also synthesized in
a circadian rhythm. It is made by many organisms, including bacteria,
fungi, and plants, as well as animals, and appears to serve protective
antioxidant functions. It is synthesized from serotonin in the pineal
gland in vertebrate animals and plays an important role in the sleep-
wake cycle.
N
H
MelatoninCircadian rhythms are found in many organisms, not just animals.
Plants, fungi, and even single-celled creatures exhibit circadian
rhythms. Among plants, the opening and closing of flowers and
leaves is a circadian rhythm. You might think that this behavior in
plants is in direct response to the daily variation in sunlight between
day and night. That is, leaves and flowers open during the day when
the sun is out and close at night when it is dark. Although sunlight-
triggered movement is part of the picture, it is not the most important
determinant. This can be demonstrated by growing the plant under
conditions of constant light or darkness. Even if a plant grows ina