Pharmacology is the study of how molecules, such as medicines, interact with the body. Scientists who study pharmacology are called pharmacologists, and they explore the chemical properties, biological effects, and therapeutic uses of medicines and other molecules. Their work can be broken down into two main areas:

• Pharmacokinetics is the study of how the body acts on a medicine, including its processes of absorption, distribution, metabolism, and excretion (ADME).
• Pharmacodynamics is the study of how a medicine acts in the body—both on its intended target and throughout all the organs and tissues in the body.

If you struggle to wake up in time for school or work or feel drowsy during a trip abroad, your circadian rhythms may be out of sync with your environment. Circadian rhythms are your internal timekeepers, and almost all organisms, from bacteria to plants and animals, have them. You can’t see them, but you can feel their effects—they control when you get sleepy, when you wake up in the morning, and when you feel hungry. Among other signals, the brain uses sunlight to keep time.