To repeat, the size of your sleep debt determines the strength of the tendency or ability to fall asleep. If your sleep debt is zero, sleep is impossible. If your sleep debt is very low, only a small amount of stimulation is required to keep you awake. If your sleep debt is very large, no amount of stimulation can keep you awake.
Think of your sleep debt as a very heavy load. You are carrying with the help of two companions. Together, the three of you can hold it up. One of your companions is pretty strong. This companion is your biological clock. The other companion is not quite so strong, and represents transient external stimulation, e.g. noise, light, excitement, anger, pain, and so on. If one of your companions drops out, you and the other may be able to manage. If both companions drop out and you are left alone, you absolutely cannot hold up the heavy sleep debt and you are crushed. In other words, you cannot stay awake no matter how hard you try. Even without external stimulation, it is usually easy to stay awake and alert if your stronger companion, the biological clock, is helping you.
With the above image in mind, it should be clear that the things we usually assume cause us to become drowsy or to fall asleep actually do not cause us to become drowsy or to fall asleep. Their true role is to unmask any tendency to fall asleep that is present already. If you believe that boredom, a warm room, or a heavy meal causes sleep, you are completely wrong! If boredom, a warm room, or anything else seems to cause you to feel drowsy, you have a sleep debt and you need to be stimulated in order to stay awake. If you frequently feel sleepy or drowsy in any dull or sedentary situation, you almost certainly have a very large sleep debt. A large sleep debt makes us vulnerable to apathy, inattention, and unintended sleep episodes. Errors, accidents, injuries, deaths, and catastrophes can be the result, not to mention poor grades.