Get yourself a pulse oximeter. (Virtually any place with a pharmacy will carry these.)
For one minute, monitor your blood oxygen saturation levels. Every five seconds, write down the reading.
Then, do the same, but record your pulse rate.
Subtract from the oxygen column the values in the heart rate column. (Regard any negative values as ones.)
Now, determine a basic rhythmic unit. An eighth note, for instance. Apply this to the numerical values you just calculated (e.g., a 2 would mean two eighth notes or one quarter note–you may decide which). You now have a rhythmic motive upon which to base your piece.
Extension: devise a way to use these numbers to determine pitch sets.