Heat, a new town, a noisy street, jet lag, or the ordinary sort of bad patch. I can usually tell at the door. Whatever the cause, four hours of sleep changes what I should be asking of you.
What actually happens on no sleep
Coordination drops before strength does, which is the dangerous combination. You can still produce the force, you are just less accurate about where it goes, and the classic outcome is landing badly in something you have done a hundred times.
Perceived effort also rises. The same class feels harder, which people read as being unfit rather than being tired, and then push against.
The instinct to exhaust yourself
Very common and mostly wrong. The theory is that a hard session will knock you out. In practice a hard session late in the day raises everything that keeps you awake and you get a second bad night on top of the first.
Hard training when rested. Not hard training to fix not being rested.
What is worth doing instead
Restorative, if I have it on. It asks almost nothing and it is designed for exactly this state.
Qigong, which can be done at whatever effort you arrived with and is largely about standing up slowly, so the coordination risk is low.
A long exhale, which you can do without a class at all and which is the one piece of self-administered breath work that is safe to recommend in writing.
What to skip
Inversions, anything at the edge of your balance, and any class where you will feel obliged to keep up. On a bad night the room is the risk, not the postures.
The line worth drawing
Occasional bad nights are ordinary and a practice helps. Persistent insomnia over weeks is a medical question, not a yoga question, and I am not going to sell you a class as the answer to it.
Come, do less, go home. On four hours’ sleep I count that as a good night’s work, and so should you.