I read with the right dream (nightmare) heart rate can climb to 180. If you slowly condition yourself I think you can exchange sleep quality for increased hear

I read with the right dream (nightmare) heart rate can climb to 180. If you slowly condition yourself I think you can exchange sleep quality for increased heart rate. But I suspect the heart needs rest too and you will die. An isometric hold would be better I think. You don't get any vo2max improvement but it does improve cardiovascular health.