According to Rebecca Ogamo Teele, an American instructor, performer and mask carver for noh, falling asleep is a perfectly respectable response to attending such plays.
In fact, she would almost recommend it.
A noh performance can be likened to a meditative state, with a rhythm forged from drum calls and breath-based vocalizations, she explains. This dreamlike world harkens back to the womb, pulsing with the mother's heartbeat.