The sensation you are feeling is probably a glimpse into sleep paralysis.
The sleep pattern starts like this :
- Hypnagogic stage - This is easy to identify. This is the stage where you are still awake, but begin to doze off and start having odd thoughts or almost small dreams or events playing in your head. Some people call these "mini dreams" or even "day dreams."
If you stay away long enough in this stage without going to sleep (or without thinking too much and being awake) you'll eventually get hypnagogic hallucinations. These are made specifically to try to "hypnotise" the brain into going to sleep. The hallucinations can be from noises, "sights" (your eyes are closed), smells, tastes, etc.
If you keep going, you will enter sleep paralysis (some people go into this regularly without choice.) This is when your brain "paralyses" the body so your body does not act out dreams. If you can stay aware in this phase of sleeping, it is described by heavy hypnagogic hallucinations and bodily sensations. Most sensations represent : floating, feelings of sinking into the bed, feelings of rolling off the bed, feeling of pressure being put upon the chest (if lying on the back.) etc. Many people mistake this part of the sleep process as being abducted by aliens, seeing angels, seeing demons, etc. You will hallucinated on top of having these body sensations.
After that is full blown dreams. If you are skilled, you can have a "lucid dream." This is when you can keep awake (better put: aware) while your body falls alseep and induce dreaming. At this point you can control your dreams (with practice). There are other ways to induce lucid dreams.
Anyways it's sleep paralysis.