To be sleepy is to be tired. You might be sleepy because you're tired from work, or you might be sleepy because you're tired from a night of partying.
A secretion is something that is released from the body in order to do something. Some body secretions are important, like sweat, while others are just waste products.
The vagus nerve is a large nerve that runs through the entire body. It plays an important role in controlling the heart and digestive system.
To theorize is to come up with a theory about something. You might theorize about the cause of a problem, or the best way to solve it.
To starve is to not have enough food. When people are starving, they are usually not getting the nutrients that they need to survive.
To sneeze is to cough violently. When you sneeze, you usually inhale a lot of air, which makes you cough.
Endorphins are substances that are produced in the brain when you are happy or when you are in pain. They are usually called the "happy hormones" because they make you feel happy and euphoric.
The limbic system is a part of the brain that is responsible for emotions and instincts. It's also the part of the brain that is most active during sleep.
To barricade something is to create a barrier to prevent people from entering or leaving. You can also barricade your home to protect yourself from attackers.