Answer:
Circadian rhythms.
Explanation:
Stimulus in biology can be defined as an obvious change in either the chemical or physical structure of an organism' environment (either external or internal). All living organisms (both plants and animals) respond to changes in their environment and as such an appropriate reaction or response is made. Also, stimulus arising from within the organism is known as internal stimulus while those from its environment are known as the external stimulus.
In organisms, the specialized cells that detect stimulus are generally known as sensory receptors while a group of these receptors is referred to as sense organ.
Since both plants and animals respond to environmental stimuli, it simply means that, these living organisms have physiological cycles referred to as circadian rhythms.
Circadian rhythms is a natural internal process that controls the physical, mental, and behavioral changes in living organisms such as the sleep-wake cycle. The circadian rhythm is simply the internal body "24 hour" clock.
For example, there are large changes in the activities that happens with respect to the organs such as kidney, liver and endocrine glands within the body of living organisms throughout the day. Also, they experience timely shift in blood pressure, amino acid levels, body temperature, metabolism, etc.
Hence, living organisms are usually more energetic and at alert when the aforementioned activities and others are at their highest point once a day as a result of circadian rhythms.
