A Principle of Yoga
Copyright 2017-Dr. Andrew Greszczyszyn
Meditation is okay to do when you are sleepy, but pranayama is warned not to be done when exhausted or sleepy.
The reason pranayama is not suited to be done when drowsy or sleepy is because of the locking function of the breath holds. If you were to fall asleep during a breath hold you’d not consciously be able to remember to take another breath and could leave in mahasamadhi.
Conversely with mediation there generally is not the muscular lock of the breathing aparatus and breathing will resume even if you fall asleep.