Final answer:
Substances associated with inducing sleep are linked to the immune system, which is intricately connected with other systems such as the nervous and endocrine systems and plays a crucial role in reacting to stress and maintaining overall health.
Step-by-step explanation:
Substances that have been isolated from the blood and have been shown to induce sleep have also been linked to the immune system. These substances are produced as a natural response to infections or stress and impact various other systems, including the nervous and endocrine systems.
One of the well-established interactions is the effect of stress on immune health, which can be felt by nearly every organ system in the body. The immune system works closely with other systems to regulate body functions and responds to changes within the body as well as external factors like sleep inducers which can be seen as part of the body's natural defense mechanism.