Passage Four
A biological clock is what tells plants, animals, and other organisms how to organize their time. This ‘clock’ helps plants and animals change their routine when they move to places with different climates and environments. Most biological clocks are based on the movement of the sun and follow a twenty-four hour cycle called a circadian rhythm. Some animals sleep only at night, making them diurnal. Animals that sleep during daytime are nocturnal. Flowers are diurnal because they close their petals at night but they open again in the morning. Bats sleep in the daytime so they are nocturnal.
Some biological clocks also follow an annual cycle, especially in animals that have annual migrations. Animals that have annual cycles do different things at different times of year. Their actions are determined by the seasons.
Different animals can live in the same area when their biological clocks make them look for food at different times of day. This process also keeps them out of each other's way. Biological clocks help plants and animals prepare for changes in the seasons. Animals need to know what time of year to reproduce so their offspring will be born when the conditions for survival are good.
Human beings' sleep cycle is a circadian rhythm, a system based on the twenty-four hour day. In the circadian rhythm most humans sleep at night. When people travel to different time zones their biological clocks have to adjust. In humans, the biological clock is in a group of cells called the suprachiasmatic nucleus (SCN.) The SCN is located near the bottom middle part of the brain.
Scientists are continuing to research the biological clock and circadian rhythms. Scientists discovered the first circadian gene in a mammal (a mouse) in 1997. It is only a matter of time before your own biological clock will tell YOU the time!

Which of the following type of animal or plant is nocturnal?